text/x-gcode

This mimetype is deprecated. You should still support it, but avoid using it to assign new mimetypes.
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.

File types

.gcode

Also appears as

Further reading

No additional links listed