highlight.js supports a very large number of languages. It would result in very poor performance if we supported all languages by default. gcode is not a default language supported, so it will not work here on Meta. You can add support for gcode on your site by adding it to the highlighted languages site setting.
For reference, here are the languages supported on Meta:
After I enabled gcode, the highlighting is still funky/ funky in a completely new way, but now it looks like highlight.js issue. I guess the gcode support is just… not working. Still better than before! just can’t handle gcode comments it seems.