First we must find the location of GtkSourceView:
$ locate gtksourceview
In my case I run Ubuntu Linux 10.04 LTS and gedit 2.30.3 and I tested the highlightingwith the default GtkSourceView, that is 2.0. I based my XML specification of the highlighting in the Java highlighting.
Finally just copy (with sudo) file alloy.lang (this is the last version) to folder /usr/share/gtksourceview-2.0/language-specs/ (in this case).
It also highlights some reserved words of extension DynAlloy, such as assume, pre, post, call and program. Don't forget to comment any mistake.
A sample screenshot. Thanks.