Goku said that the only way to beat Gohan-Boo would be to use the Potara.
Chapter: 503 (DBZ 309), P8.8
Context: Goku tries to get Vegeta to use the Potara with him, but Vegeta refuses
Goku: “Knowing you, I thought you might say that…! There ain’t any other way to beat Majin Boo!â€
It wasn't until Boo turned back to regular Evil Boo that Goku suggested the regular fusion.
Chapter: 506 (DBZ 312), P13.1
Context: still talking about how they’re no match for Boo
Goku: “…But there is one way we can win!â€
Vegeta: “You want to say Fusion, right? Well who cares about that?!â€
Goku: “Huh? You know about it?â€
Vegeta: “I saw it from the afterlife…You’ve got to be joking! You think I’d perform those ugly poses…?! Anyway, I thought I told you that I’m not going to merge with you a second time.â€