Okay. I've been waiting for this match up too, lol. It seems really niche.
First thing I want to establish is Gash's feat. It's his first, and weakest spell at a time when it was at its weakest as it was just cast regularly without even being augmented by heart power. This is before Gash fights foes like Bari, who can nope Zakeru with no damage, punch through Zakeruga, and tank their one shot, Bao Zakeruga albeit with a shield.
And Gash's fight with Bari doesn't actually mark the end of the arc. Not that Gash is as strong as Bari in the beginning, but seeing as how his first spell is the weakest one, and it was done pretty early on in the series, I think it's safe to say that the feat is casual enough for Gash to keep his 2x AP and dura advantage. Gash at the end of BoS should be only a bit below Bari (Note, I'm assuming Bari also got stronger during that time).
Also, not sure if this is listed, but compared to their mid-level spells, demons have higher durability than their AP (If they're scaled to their mid-level spells or lower). As demons can tank low and mid tier spells with varying levels of difficulty, but with rather extreme consistency. So a 7/8 may not even OHK Gash regularly. A 9 likely, but demons have really solid stamina and dura, being able to use their body to resist elements that would normally be impossible with just dura alone. Things like flame, gravity, etc. Iirc, Gash is noted multiple times to have a particularly strong body.
I also doubt that duplicates will be an issue. Like Gash did against Kid, he can literally run about the battlefield with Rauzaruku, and throw them hundreds of meters away, making them fairly irrelevant to the battle at hand. Not to mention that something like Rashield would essentially make all of G&W's ranged attacks a bit useless unless he can maneuver them around the shield.
Kiyomaru is also insanely intelligent, and catches onto things rather quickly. He's no stranger to abusing range against someone who has to get close, and will do things like use Zakeru to kick up dust to blind his opponents, cast spells purely for distraction purposes, determine how an opponent's ability functions after seeing it once or twice, etc.
The battle taking place near the white house also means that there should be some kind of metal around. And since G&W has less AP than Gash, Jikerudo into a Bao Zakeruga in the midst of the match would constitute a one shot, which I don't think G&W really has any answers for. Jikerudo will even pull G&W down if he's flying. Though flying is really only useful for mobility here since his range is inferior.
Also, if he pulls out his wings he runs the risk of them being pierced cleanly by Zakeruga (which seems to move faster than Zakeru).