No, I'm not a fanboy. I'll stay objective and explain why I think SSJ Gogeta would win.
SSG Gogeta and SSJ Vegito defeated Buuhan and Janemba, both characters are well above SSJ 3 level, and oh man, SSG Gogeta and SSJ Vegito defeated them both with such ease that it was painful to watch.
Goku and Vegeta had an impressive training with Whis, back then there was still this "Beyond God Saiyan" who was nothing more than an SSG. But the producers thought that the SSJ transformations would still sell well, and power scaling would be a problem, so they gave Goku and Vegeta the SSJ transformation, and the Beyond God thing was scrapped. But it doesn't matter, we're just concentrating on the film. Goku has his Beyond God form which is as strong as an SSG. Frieza Final Form was significantly weaker than Goku. So I think Frieza is between SSJ3 and SSG. I honestly don't think there's that much of a difference between SSG and SSJ3.
If we know that Frieza is at SSJ3 and SSG level and Janemba and Buuhan are in between, then we can compare them very well. And as I said, Gogeta and Vegito defeated both of them effortlessly, so I think Gogeta is stronger.
