I've done a quick .lang file to allow BeemASM .asm files to appear correctly highlighted in the GNOME text editor Gedit.

If you are using GNOME 3, then the .lang file needs to go the following folder for just one user:
.local/share/gtksourceview-3.0/language-specs/
or to allow everyone to use it put it in:
/usr/share/gtksourceview-3.0/language-specs/

If you are using GNOME 2 then it's the same as above but
.local/share/gtksourceview-2.0/language-specs/
to store it locally for one user or to saddle everyone with it:
/usr/share/gtksourceview-2.0/language-specs/

If you go to View->Highlight Mode->Sources in Gedit you'll see a new entry BeebAsm. This highlighting should be applied automatically if your file has a *.asm file-name.

Don't worry if the folders in .local don't exist on your computer - that's normal. Just create them before adding the file) and of course to see hidden folders you need Ctrl+H

EDIT: Everyone apart from me probably already knew this, but you may find it helpful on Linux to put a shortcut to the beebasm binary in the ./local/bin/ folder - that means the current user can then enter beebasm ... directly from the terminal instead of having to type ./beebasm/beebasm ...