So, since this is a scenario where both are aware of each other's moves. Here's what I believe is most likely to happen.
Because of kens incredible ability to perfectly memorize and counter any martial art once, gill will have the hardest time trying to hit Ken at all or even try to counter him. Once Kenshiro has fought or seen how an opponent fights or how their style works, he literally creates new counter moves for that opponent (in every fight he had to face the same opponent twice, he dominates them because he knows how to counter their every move.
While gill will be definitely on the defense, I'm very sure that Ken will catch him and defeat him because, his incredibly hax moves will eventually hit gill (even blocking one of kens attacks will lead to the opponent exploding, look at Tenha kassatsu and the go no Ken techniques)
Now of course gill has some advantages like AP and dura going for him. But the real edge gill has is his resurrection abilities.
Now resurrecting from a soul destroying move is insane. However this isn't like he could continually spam resurrection and do that. also while gill did survive and revive from akumas attack, this just means he has strong soul destroying resistance, I'm not even sure he could continue the fight since his body would sorta be in a bloody pool, and I've never seen gill regen his body from either being blown up or cellular vaporization(sincere is labeled to have low mid regen) so Ken will win form this, this is one way you could look at the outcome.
Now let's just say he could revive his body somehow from his body exploding and is at full health. Ken will be very much supposed by this fact and either two things will happen.
Gill will try to hit him with seraphic wings or something(but kens knowledge of said move, combined with precog Will make him dodge it or block) and Ken would most likely go museo tensei, which will put gill by complete surprise as he sees all of his attacks go through him, and once Ken goes near him he could just erase gill from existence(most don't know this but museo tensei has been defined as
being able to erase a persons body from existence]
So aside from superior durability and AP, and the ability to ressurect(to clarify he can't continually revive himself with this and only has one chance to come back, saying he could comity is a lie since the gameplay shows he can't) their isn't much gill could do here.
Having knowledge of kens move sets won't change much on how he could fight back, just he'd be more distant and careful. However Ken also knows gills move set and has great combat intuition, as he realizes gill is staying away for a reason. so Ken will adapt to this as he has plenty of haxes ranged moves.
Combined with kens precog and immense skill (which from what I know of both fighters) that trumps gills. Not to mention kens overwhelming hax like MT. Ken takes this low mid difficulty