In the beginning of Resurrection F, Whis said after training Goku and Vegeta in SSB that if they found a way to work as one they "could go toe to toe with Lord Beerus". Vegeta reacts in the manner that suggest Whis means fusion by said "we did that once, it'll never happen again".
In the Zamasu arc when they fuse to become SSB Vegito, Supreme Kai says they may even be stronger than Lord Beerus.
In the DBS: Brolly movie after fighting Brolly as part of SSB Gogeta, Goku tells Brolly that he may even be stronger than Lord Beerus.
What I'm getting at with this, is that either directly or indirectly, a Goku and Vegeta fusion as Super Saiyan Blue is somewhere in Beerus ballpark. Beerus gets nailed by Whis and Whis states that he's nothing compared to the Grand Priest. We've already been told that neither Goku nor Vegeta can actually get more powerful, that their training is about ki control and efficiency, so in all honesty, no, I cant see Goku ever approaching the Grand Priests power.
Gogeta or Vegito using Ultra Instinct on the other hand.... Could maybe give Whis a fight.