There's going to be a pier in GC eventually, they have already made the commitment. Not sure how many ships will be able to use it, but I suspect that it will be similar to Cozumel. CCL ships dock at the pier (Costa Maya) they gave money for and RCCL docks at the one they helped build. BTW--I understand that Punta Langosta in Coz has now been sold so there are no longer any private cruise piers there. I haven't been able to find out who bought it though. NCL had used it for years.
I would think there are too many ships for all to be able to dock in GC.