Some compilers want an underscore, others don’t. Here is how I fixed it, better looking suggestions welcome…
I extracted this from a gnome bugzilla entry. I extended the file a little to include some 64-bit registers and instructions. Install e.g. by copying to
/usr/share/gtksourceview-2.0/language-specs/ (there might also be a way to use your gedit config in your home directory).
I don’t maintain this for newer versions of Gedit since I use Sublime Text now.