Goku suggested fusion with Gohan when Gohan was actually being attacked by Boo, so he seems to think it will be easy to perform, even for someone who never saw the dance. Old Kaioshin tells him it will fail because Boo won't let them do it, but not because Gohan would need to learn it first. Take Vegeta who has actually seen the dance performed. Goku and Vegeta had ample time to perform fusion while Gohan Boo was approaching them, yet the only way that they could beat him was through Potara.