Gamabunta he's much faster than Guts and durable enough to tank any of his hits, where as one shot from Gama would basically cripple or kill Guts
im going to go ahead and assume you dont know how the berserker armor works, or who guts is
as far as the fight goes, I say guts takes this Mid difficulty
I know who Guts is and how the Beserker Armour works he's not fast enough to fight Gamabunta who is MHS at the least while he's somewhere around the HS+ range, hes not durable enough to tank Gamabunta's hits who was able to damage the 1 Tails and he's not strong enough to hurt Gamabunta either.
In all honesty Guts is more suited for a match-up against KN0 Naruto they are both in the same speed tier and both have similar strength tho Naruto is slightly better.