This type has been deprecated in favor of one of it's alternatives. While you should likely still support it, it should not be your first choice if assigning a MIME type.
Instead please use gcode

GCODE files are created by slicing programs, such as Simplify3D and Slic3r, that translate CAD drawings into G-Code, which a 3D printer can read.

There's no official mimetype assigned as far as the author is aware, however these are the ones commonly reported.

Used in file types
  • .gcode
This type may also appear as
  • text/x.gcode
  • gcode
Further reading

No additional sources listed for this type