How big is the gap between GM Luke and DE Sidious.

It's almost unilaterally accepted that Luke surpassed Sidious but by how much?

It’s very difficult to say, but I don’t think by a massive degree

In overall power? Likely quite a bit. Luke has had enough time to actualize a large portion of his insanely high potential; it was already enough by DE so that with elementary battle meditation from a barely trained Leia, he was able to overpower Sidious (in saber combat, but explicitly through superior power). Luke has had decades to grow since then, and he tends to grow very quickly. The gap in sabers is likewise probably large.

In offensive Force abilities? That's where things get murkier, given Sidious's potentially unprecedented breadth of dark side powers, including Force storms. Unless if it's a context where Sidious's Force storms dominate (assuming Luke can't Force harmony or otherwise counter them - which he likely can, given that he and Leia pulled it off in DE when they were both collectively much weaker than prime Luke), Luke probably still has the advantage, especially since he isn't lacking in Force abilities himself (e.g. emerald lightning), but it's much closer. In situations like dealing with large numbers of enemies at once, Sidious is likely more potent.

The caveat here is that I'm referring to an unleashed, all-out Luke, e.g. at parts against Abeloth. That Luke probably has a very large, but not ragdoll-tier, advantage over Sidious. The Luke that fights UnuThul and in TUF (before his absolute prime) probably beats Sidious with high-difficulty. If Luke fights the way he does against random villains like the Hidden One, he isn't beating Sidious.

Peak Luke - 200
vs. UnuThul Luke - 170
TUF Luke - 160
DE Sidious - 150
Caedus-fight Luke - 120
{If TPM Maul were a 200, Qui Gon would be like a 185. If ESB Vader were a 200, ESB Luke would be like a 150}