I have a .650 GT, and a .665 GT for my 870.
My belief is that .655 or .660 is probably the best bet with a GT in a RemChoke gun.
.650 is a hair too tight, and .665 is a hair too open with a GT.....compared to the patterns I get from my Indian Creek .665 and .675 chokes in the same 870.
It seems like a little extra constriction is better with a GT choke. Nice choke, just go with a tight one.