Speed is equalised firstly and Ruby has never been so fast she cant be tracked or timed (like Mercury), it takes literally a thought to set a barrier with whatever is around him, and Ruby is unlikely to start off using her semblance to suddenly charge him since as we all know, Ruby forgets her semblance most of the time between V1-3 that the show itself makes her acknowledge it more afterwards. She also wont be able to deal effective damage in this form and Gentle himself can use his quirk to amplify his movement while hindering hers.
Yeah, they can, since they're literally invisible and Ruby from what we've seen cant really see whats happening while in this form, or split herself since thats past V3 to get around them. Unlike her, he's experienced at fighting with his barriers present, and has a whole air sky space he can travel around rather than thinking he's dumb enough to keep himself trapped and that he would only be stuck in one position throughout the battle. Even if he did do that, Ruby couldn't touch him regardless.
She obviously does since theres no other viable way she can hurt him consistently seeing as her ranged options get rebounded, shes useless with her fists, her scythe can easily become unwieldable etc. Shes stuck colliding into him somewhat, which as shown by Mercury, can also be hit and rebounded itself with minimal effort. If Gentle hit a wall, he would probably use his quirk to weaken the impact and launch him back into the battle to attain that key close distance against her. The point is, Ruby is forced into close quarters and thats not gonna be good for her since Gentle can literally disable her weapon capabilities.
The fight won't last long. Gentle can continually use his quirk with very little strain, and Ruby has no idea in the first place whether it can even turn back or how it works. It isn't at all related to the air barrier argument, and given the fact Gentle's fight with Midoriya had an actual short timescale (Especially with most of the time Gentle trying to escape), i dont see whats literally so wrong with saying this fight wont last ages. This is all fictional. And perhaps i could respect Ruby could figure out that Gentle has these barriers if she could literally do anything against them? What is she gonna do? Be unable to approach him and try to outsmart Gentle about his own quirk that she only just figured out? This isn't the only application of his quirk either.
Gentle would win by being able to literally counter all her abilities except short burst speed (that has its own time limit and will keep chipping her aura) and disable her ability to fight back. Gentle knows how to fight, he can take on Pro Heroes (that in verse are far more competent than the graduated hunstmen in RWBY), and can easily exhaust her while shes on an aura limit. You're very much underestimating Gentle if you think he cant do anything combat wise to take out an opponent while his versatile ability easily keeps her advantages in check.