As it seems there is some demand for my ActionScript3 language file. So I will make it available as download here. Please, be aware that this file is not perfect, and you may want to edit this file. To use this file within GEdit, just copy the file to the folder /usr/share/gtksourceview-1.0/language-specs. Restart Gedit and you can select the Highlighting format in GEdit.
If you want GEdit to automatically switch to the ActionScript3 syntax whenever you open an .as file, you need to associate an MIME type with the .as files.
Get the language file here.