Grunbeld VS Guts @ Full Health

If Slan didn’t nearly kill Guts in their last encounter, how would this fight go?

Guts would still lose. Berserker Guts got his arm twisted from taking one hit from Grunbeld and even base Grunbled was stated to surpass most transformed apostles, who still give this Guts a tough time. Maybe Guts might be able to do some damage with his cannon but there's no way he'd win. In fact I think even Berserker Guts would've lost against Grunbeld eventually with how the armor was digging into his limbs and how much blood it made him lose. The only damage Grunbeld had taken by the end was one crack to his armored skin.