First if they fight on horses I pity the poor lad who will have to carry that huge man and his huge weapon.
Mobility wise I don't think GKHK can really give his best on horses. His style is a lot on dodging and dancing and mounted it will be a lot more difficult to swing and dodge especially against someone who is used to to that. And he cant really kick and punch while on horses.
So on horses I give the win to Gyou'un without too much thinking.
On foot it will be a lot closer : Gyou'un will only use a glaive, we never saw him using anything else so lets say he will keep his glaive to fight.
GKHK with his martial art can parry Shin glaive with ease and also with his mobility can land a lot of hits and send SHin back. GKHK hits are strong but Shin still managed to block them. It means they werent overwhelmingly strong either.
But 2 things : if Gyou'un keeps using his glaive, GKHK won't be able to switch with his swords. It is really ill advised for him to do that because he won't be able to block Gyou'un heavy glaive only with a sword (and we saw how it ended in the last chap if he tried to block glaive with a sword). And if GKHK keeps his huge boulder/mace he wouldn't be able to use his kicks and punchs. The boulder is likely too heavy to do both and even if some times we saw GKHK carrying the boulder with one hand, he mainly needed his two hands to hit Shin with blows or parry.
So Gyou'un will have a really hard time landing a hit on GKHK and will have to parry strong blows. But if Shin has the strength and skills to to it Gyou'un can at least do it either.
Other thing : Shin said that the glaive was too heavy to handle GKHK speed. Shin is really really strong physically. But Gyou'un was fighting with his glaive all his life, is 100% used to it, its weight etc, and I think in sheer power he can have a little edge over Shin. Gyou'un is also really skilled. So i think he can keep up a bit better than Shin against the boulder. And Gyou'un has weight. Shin against GKHK was a bit angry but wasn't enraged or really riled up. We saw Shin giving harder hits than that in the past. Gyou'un blows will be heavy.
And frankly portrayal wise Gyou'un seemed to be prime choice of vassals under a better GG and his presence and fights were more impressive than GKHK.
I give him the fight even on foot but he will sweat and will need to give his A game.