GMAB is Green Monster Assault Battalion, and it was a true assault (all assault mechs) not just a heavy battalion called assault if I remember correctly?
I don't think Regulus makes any assaults right now? Lots of good heavies, but no assaults. So I'd look for refits or old FWLM standards. Stalker (6M), Awesome (9M, 9Q), Battlemaster (3M, 5M), Goliath, Sirocco, Grand Titan, Thug (10E), Longbow (12C?), Emperor (6A).