Then that should be enough to overcome Gregor. True, he's faster than people in-universe give him credit for, but that doesn't really mean anything other than most people think he's big and slow.
Wouldn't Guts still be faster than Clegane? Clegane might win in a grapple, but Gut's sword was still heavy enough to hit like a warhammer. Actually, I think comparing young Guts to peak Bobby B isn't too far off.