Super Saiyan 4 Vegeta and the turtle that lives with Roshi.
Gogeta's power level = 999,999,999,99...
Turtle's power level = 0.001 (said by Raditz's scouter)
so if fusion is a addition problem then Gogeta's power level becomes 0.001 higher. And if fusion is a multiplication problem, then Gogeta is pretty much screwed.