10/2 with ground I presume?
Safety wise, it is fine to use a larger than required wire.
Mechanical wise, a #10 should fit in the breaker OK (look at specs on breaker), but might not in the GFCI. The new GFCI's I've seen have a hole in the back for the wire and you tighten the connection with the screw on the side - can't wrap the wire around the side and I don't think you would want to try that with a #10 anyway. But the hole on one I have here looks like #12 is the largest it will accept, but mine is a 15 amp. Maybe a 20 amp GFCI would accept a #10? I don't know?
I don't see wire sizes when looking at the GFCI specs on the internet.
Then cramming everything into the electrical box wise... I would use a deep double gang box with a single gang plaster ring.