Hi Nige
Yes been a couple of times - once in April and once in October. Gangga itself is terrific - really upmarket and pretty expensive. Food is brilliant and too much for me! There is a great camera room.
Diving wise it is all small stuff and I never saw serious shoals of anything. Too much fishing in the area for that. It differs from Bunaken in that there are only a few steep walls but what there are are very scenic. I hardly use my wide angle when there. Coral is generally in good order but I don't recall serious amounts of soft coral. There is also a good muck diving area on the N. Sulawesi coast about 30mins from Gangga.
Diving is well organised but you have to be supervised which I hate. Depending on who your supervisor is (you are allocated one for your whole stay), you could be limited to 60mins flat and have to surface with half your air left - or do your own thing and then tell your guide when you are ready to surface. Like other places out there, all the guides are great critter spotters but can be a bit too enthuisiastic (with tips in mind I suppose) and can seriously muck about with subjects in order to present them to you unless you let them know you don't want that sort of thing. Nitrox is available.
PM me if you want further info