The best I have seen as GDI has been a balanced mix of a few people keeping the tunnels under control, a good sniper/counter-sniper, and mainly several med tanks and a couple of MLRS's. Good eng support helps a lot too. It's nice if you can keep control of the waterfall overlook, but not neccesary - as long as your tank drivers keep an eye out for enemy fire coming from the waterfall - Raveshaw can give you grief from there. A few hits from the MLRS's seem to discourage them.

Key in the long run is to keep the field under your control. Rushes work if you can catch them off guard - if they are concentrating on the tunnels - they are probably light on tanks and defense.

I'd forget about the mammies unless and until your are right up to their end of the field, and then you should have good eng support.

Have fun!