It's stated in the GT Perfect Files 2 iirc, or that Gogeta is several times stronger than Goku and Vegeta's combined power.
Whatever the case, the more important indication of their gap is given in the actual series. After absorbing the Dragon Balls, Yi Xing Long states his power has increased tenfold, maybe even more, with his initial power not being much lower than regular stomp territory in relation to Beyond Limits SS4 Goku. This proves that at bare minimum, SS4 Gogeta is easily over 10x stronger than Beyond Limits SS4 Goku and Vegeta.