I mean, a Gamma Ray Burst by itself means nothing. It's the most energetic phenomenon in the universe simply because of the sheer magnitude of mass expelled and the forces involved- in simpler words, an artificial GRB would not be as strong as a natural one if it was not as large as a natural one. It's like comparing a nuke to the sun just because both work on the same principle
The GRB in One Punch Man was smaller than a planet, it took place in one. There's no reason to think the energies involved come anywhere close to a real one, especially when the manga goes out of the way to say it follows the same physics as a real one. Someone calculated the feat in VSB, and it comes at large planet level.
It was Serius Punch Squared, as the name implies- the forces here are being squared and it's not something either one of them can do alone without resonance. Even a moon level attack becomes multi-star level when its magnitude is squared. Plus, Garou losing his shit over Saitama sneezing away Jupiter contradicts them being star level.
That being said, Blast and Empty Void can also solo Invincible as far as I know, so it's not just Saitama and Garou alone who can survive. Then there's God, of course.