At what point exactly...?
When Kid Buu went to Kaioshin's planet? Gogeta would defeat him in a blink of an eye.
After the SSJ3 plan failed? It's relative. Even then, varying the continuity (wheter it's Toei or manga), if it were for Toei, I think they would allow Gogeta to transform into SSJ, as Goku & Vegeta were heavily damaged by Janemba, but their fusion went SSJ since the very beginning.
If we talk about manga continuity, I don't know, everything could happen and perhaps Gogeta is the one that make the Genkidama instead of Goku while Good Buu distracts him, even then, I don't think Gogeta would actually need to transform into SSJ to overpower him with it after having his full stamina restored unlike Goku. Even then, having two fusions of Goku & Vegeta in one saga seems unlikely, but a good idea too.