CalculiX is a free software nonlinear three-dimensional finite element program for structural applications. Following steps results in syntax highlight of inp file in gedit:

1. Add .xml in ~/.local/share/mime/packages named ccx.xml with following content:

    <mime-info xmlns='http://www.freedesktop.org/standards/shared-mime-info'>
     <mime-type type="text/x-inp">
       <comment>CalculiX CrunchiX inp files</comment>
       <glob pattern="*.inp"/>
     </mime-type>
   </mime-info>

2. Update mime-database as:

          $ cd ./local/share
          $ update-mime-database mime     

3. Add ccx.lang file in /usr/share/gtksourceview-3.0/language-specs or in path resulting from following command:

           $ locate gtksourceview | grep /c.lang
   

   The content of ccx.lang is:

   < ?xml version="1.0" encoding="UTF-8"?>
   <language id="ccx" _name="CalculiX" version="2.0" _section="Sources">
     <metadata>
       <property name="mimetypes">text/x-inp</property>
       <property name="globs">*.inp</property>
     </metadata>
     <styles>
       <style id="comment"           _name="Comment"               map-to="def:comment"></style>
       <style id="keyword"           _name="Keyword"               map-to="def:keyword"></style>
       <style id="card"              _name="Input Cards"           map-to="def:type"></style>
       <style id="element"           _name="Element Library"       map-to="def:constant"></style>
       <style id="floating-point"    _name="Floating point number" map-to="def:floating-point"></style>
       <style id="decimal"           _name="Decimal number"        map-to="def:decimal"></style>
     </styles>
     <definitions>
       <context id="comment" style-ref="comment">
       <start>\*\*</start>
       <end>$</end>
       </context>
       <!-- http://www.lysator.liu.se/c/ANSI-C-grammar-l.html -->
       <context id="float" style-ref="floating-point">
         <match extended="true">
           (?&lt;![\w\.])
           ((\.[0-9]+ | [0-9]+\.[0-9]*) ([Ee][+-]?[0-9]*)? |
            ([0-9]+[Ee][+-]?[0-9]*))
           [fFlL]?
           (?![\w\.])
         </match>
       </context>
       <context id="decimal" style-ref="decimal">
         <match extended="true">
           (?&lt;![\w\.])
           [0-9]+[uUlL]*
           (?![\w\.])
         </match>
       </context>
       <context id="keywords" style-ref="keyword">
         <keyword>2D</keyword>
         <keyword>3D</keyword>
         <keyword>CIRC</keyword>
         <keyword>ELSET</keyword>
         <keyword>MATERIAL</keyword>
         <keyword>NAME</keyword>
         <keyword>NSET</keyword>
         <keyword>OUTPUT</keyword>
         <keyword>RECT</keyword> 
         <keyword>SECTION FORCES</keyword>
         <keyword>SECTION</keyword>
         <keyword>S</keyword>
         <keyword>TYPE</keyword>
         <keyword>U</keyword>
       </context>
       <context id="elements" style-ref="element">
         <keyword>C3D8</keyword>
         <keyword>F3D8</keyword>
         <keyword>C3D8R</keyword>
         <keyword>F3D8R</keyword>
         <keyword>C3D8I</keyword>
         <keyword>C3D20</keyword>
         <keyword>F3D20</keyword>
         <keyword>C3D20R</keyword>
         <keyword>F3D20R</keyword>
         <keyword>C3D20RI</keyword>
         <keyword>C3D4</keyword>
         <keyword>F3D4</keyword>
         <keyword>C3D10</keyword>
         <keyword>F3D10</keyword>
         <keyword>C3D6</keyword>
         <keyword>F3D6</keyword>
         <keyword>C3D15</keyword>
         <keyword>F3D15</keyword>
         <keyword>S3</keyword>
         <keyword>S4R</keyword>
         <keyword>S6</keyword>
         <keyword>S8</keyword>
         <keyword>S8R</keyword>
         <keyword>CPS3</keyword>
         <keyword>CPS4</keyword>
         <keyword>CPS4R</keyword>
         <keyword>CPS6</keyword>
         <keyword>CPS8</keyword>
         <keyword>CPS8R</keyword>
         <keyword>CPE3</keyword>
         <keyword>CPE4</keyword>
         <keyword>CPE4R</keyword>
         <keyword>CPE6</keyword>
         <keyword>CPE8</keyword>
         <keyword>CPE8R</keyword>
         <keyword>CAX3</keyword>
         <keyword>CAX4</keyword>
         <keyword>CAX4R</keyword>
         <keyword>CAX6</keyword>
         <keyword>CAX8</keyword>
         <keyword>CAX8R</keyword>
         <keyword>B31</keyword>
         <keyword>B31R</keyword>
         <keyword>B32</keyword>
         <keyword>B32R</keyword>
         <keyword>D</keyword>
         <keyword>GAPUNI</keyword>
         <keyword>DASHPOTA</keyword>
         <keyword>SPRINGA</keyword>
         <keyword>DCOUP3D</keyword>
       </context>
       <context id="cards" style-ref="card">
         <prefix>\*</prefix> 
         <keyword>AMPLITUDE</keyword>
         <keyword>BEAM SECTION</keyword>
         <keyword>BOUNDARY</keyword>
         <keyword>BUCKLE</keyword>
         <keyword>CFLUX</keyword>
         <keyword>CHANGE FRICTION</keyword>
         <keyword>CHANGE MATERIAL</keyword>
         <keyword>CHANGE PLASTIC</keyword>
         <keyword>CLOAD</keyword>
         <keyword>COMPLEX FREQUENCY</keyword>
         <keyword>CONDUCTIVITY</keyword>
         <keyword>CONTACT FILE</keyword>
         <keyword>CONTACT OUTPUT</keyword>
         <keyword>CONTACT PAIR</keyword>
         <keyword>CONTACT PRINT</keyword>
         <keyword>CONTROLS</keyword>
         <keyword>COUPLED TEMPERATURE-DISPLACEMENT</keyword>
         <keyword>CREEP</keyword>
         <keyword>CYCLIC HARDENING</keyword>
         <keyword>CYCLIC SYMMETRY MODEL</keyword>
         <keyword>DASHPOT</keyword>
         <keyword>DEFORMATION PLASTICITY</keyword>
         <keyword>DENSITY</keyword>
         <keyword>DEPVAR</keyword>
         <keyword>DFLUX</keyword>
         <keyword>DISTRIBUTING COUPLING</keyword>
         <keyword>DLOAD</keyword>
         <keyword>DSLOAD</keyword>
         <keyword>DYNAMIC</keyword>
         <keyword>ELASTIC</keyword>
         <keyword>ELEMENT</keyword>
         <keyword>ELEMENT OUTPUT</keyword>
         <keyword>EL FILE</keyword>
         <keyword>EL PRINT</keyword>
         <keyword>ELSET</keyword>
         <keyword>END STEP</keyword>
         <keyword>EQUATION</keyword>
         <keyword>EXPANSION</keyword>
         <keyword>FACE PRINT</keyword>
         <keyword>FILM</keyword>
         <keyword>FLUID CONSTANTS</keyword>
         <keyword>FLUID SECTION</keyword>
         <keyword>FREQUENCY</keyword>
         <keyword>GAP</keyword>
         <keyword>GAP CONDUCTANCE</keyword>
         <keyword>HEADING</keyword>
         <keyword>HEAT TRANSFER</keyword>
         <keyword>HYPERELASTIC</keyword>
         <keyword>HYPERFOAM</keyword>
         <keyword>INCLUDE</keyword>
         <keyword>INITIAL CONDITIONS</keyword>
         <keyword>MATERIAL</keyword>
         <keyword>MODAL DAMPING</keyword>
         <keyword>MODAL DYNAMIC</keyword>
         <keyword>MODAL CHANGE</keyword>
         <keyword>MPC</keyword>
         <keyword>NO ANALYSIS</keyword>
         <keyword>NODAL THICKNESS</keyword>
         <keyword>NODE FILE</keyword>
         <keyword>NODE OUTPUT</keyword>
         <keyword>NODE PRINT</keyword>
         <keyword>NODE</keyword>
         <keyword>NORMAL PRINT</keyword>
         <keyword>NORMAL</keyword>
         <keyword>NSET</keyword>
         <keyword>ORIENTATION</keyword>
         <keyword>PHYSICAL CONSTANTS</keyword>
         <keyword>PLASTIC</keyword>
         <keyword>PRE-TENSION SECTION</keyword>
         <keyword>RADIATE</keyword>
         <keyword>RESTART</keyword>
         <keyword>RIGID BODY</keyword>
         <keyword>SELECT SECTION</keyword>
         <keyword>SOLID SECTION</keyword>
         <keyword>SPECIFIC GAS CONSTANT</keyword>
         <keyword>SPECIFIC HEAT</keyword>
         <keyword>SPRING</keyword>
         <keyword>STATIC</keyword>
         <keyword>STEADY STATE DYNAMICS</keyword>
         <keyword>STEP</keyword>
         <keyword>SUBMODEL</keyword>
         <keyword>SURFACE</keyword>
         <keyword>SURFACE BEHAVIOR</keyword>
         <keyword>SURFACE INTERACTION</keyword>
         <keyword>TEMPERATURE</keyword>
         <keyword>TIE</keyword>
         <keyword>TIME POINTS</keyword>
         <keyword>TRANSFORM</keyword>
         <keyword>UNCOUPLED TEMPERATURE-DISPLACEMENT</keyword>
         <keyword>USER MATERIAL</keyword>
         <keyword>VALUES AT INFINITY</keyword>
         <keyword>VIEWFACTOR</keyword>
         <keyword>VISCO</keyword>
       </context>  
       <!--Main context-->
       <context id="ccx">
         <include>
           <context ref="gtk-doc:inline-docs-section"></context>
           <context ref="comment"></context>
           <context ref="float"></context>
           <context ref="decimal"></context>
           <context ref="keywords"></context>
           <context ref="cards"></context>
           <context ref="elements"></context>
         </include>
       </context>
     </definitions>
   </language>

4. Open inp file and select gedit program to open.
5. If you dont like or face problem then revert back the inp files to plain text as:

       $ gvfs-set-attribute [path_to_your_inp_file]  metadata::gedit-language ''
   

Syntax Highlight Files can be downloaded from below link:
Download Syntax Highlight Files.

Fig.1: Before Syntax Highlight of CCX input file.

Fig.2: After Syntax Highlight of CCX input file.

Reference:

1. Gedit New Language File
2. Language Definition v2.0 Tutorial
3. SuperUser.com
4. CalculiX CrunchiX