Yes, C and its similars can only be licensed through GI and you will have to keep them with GI for the duration of your contract.
Otherwise, you are a free man and you can work with other agencies to license non-similars.

Anyway, maybe it helps to repeat: my personal experience showed that  spreading the same photos over multiple agencies produces ~30% more than what GI can do.

Sent from my SM-N910T using Tapatalk