Can we add those?
AFAIK it’s just a matter of dropping an xml file into /usr/share/gtksourceview-[Version].0/language-specs/
There’s a GPL NASM syntax here:
http://callumscode.com/downloads/nasm.lang
Unknown Licence for ATT on the gnome bugtracker:
https://bug152267.bugzilla-attachments.gnome.org/attachment.cgi?id=45158&action=view