Here, I'm giving you a little spoiler. Let me tell you—don't think that Goku is going to be stronger than Beerus in Super Saiyan 4 God form, or even in Blue form, not even when combined as Gogeta (Goku and Vegeta fused). Why? Isn't it obvious? I mean, this is the upper world—it's not the same as Super, but a higher level altogether.
The second thing is simple: Goku is unstable because of his soul. Here's the spoiler—the memories of GT Goku have returned to the current Goku. The soul of GT Goku still exists inside him. So, he's not using his full power.
Now, let's look at Beerus. He's been alive for millions of years. That man has destroyed his own world, his wife, his love, his planet, and countless other planets. He has fought countless beings across the 65 million years of his life—and maybe even more. Why? Because he has also been sleeping for around 35 million years. Then, when he woke up, he simply tasked Frieza to destroy Planet Vegeta.
So how can Goku—who has only been alive for about 30 years—compare to someone who has been fighting, destroying, and surviving for tens of millions of years?