Can a Gen 1 Pokemon from g/s/c be traded back to b/r/y with a Gen 1 move that it can't learn in b/r/y? For example: A Machamp that learns Fire Punch in g/s/c being traded back where there is no way it can learn the same TM in b/r/y.
Your Machamp would most likely be able to be transferred back to your copy of RBY without any issues, provided it doesn't know any moves that don't exist in Gen 1. Fire Punch is a special case because it exists in RBY, while other moves like DynamicPunch would make your Machamp glitchy. If you're looking to transfer it backwards, make sure it knows no moves that are only available from Gen 2 onwards, otherwise it could be very dangerous to your game.