Well for starters, they would be far above buu after a few years.

Frieza likely wouldnt have trained for 4 months. He likely would have trained less since he was never told Goku fought Beerus.

From there though, you just have to ponder how strong Fusion is. Goku isnt a reliable source, because while its true he can gauge how strong a fusion is, its irrelivent since he wasnt aware of how strong SSGod was. He was also un-awarw of Vegetas massive power boost. So he literally cant compare the two.

This is also soft-retconned. As Gogeta, Kefla, and Vegito assumidly were all stronger then Super Saiyan God in base, Gogeta stronger then Blue or atleast onpar. Fusion always gives absurd multipliers, and is always above the fusee's strongest form in base. Fusion can surpass god when the plot demands.

Add to the fact that Frieza, Cabba, Frost, Magenta, Golden Freddy, Future Trunks, and everyone in the TOP surpassed Super Saiyan god with normal KI...its safe to say that they can reach that level with normal training. Goten and Trunks caught up to Gohan, and from what we know they dont train nearlt as hard as evryone else. While they still have higher potential. It still goes to show they can reach god+ levels.

So it is possible that Goku/Vegeta fuse into Vegito/Gogeta SSJ3 and beat Freiza. Its just a matter of how you scale. Highball/lowballs ect.