He's definitely Mid-Kage level but High-Kage is debatable.
Out of the current Kage, he could beat Gaara, Tsunade and Mei but Ay and Onoki is heavily debatable.
Gai not beating Gaara lol you're basing that on Lee vs Gaara, When Lee said what Gaara can do vs Kimmimaru it was pretty obvious if they fought again that Gaara>Lee in part 1. Now Gaara is even stronger than he was back then and can create and control a vast amount of sand that he can use to reinforce his defense plus he can fly high above Gai's reach while attacking him from below. Just standing in 1 place would mean a lost since he would be standing on sand (Gaara can make sand from the ground) and get his legs sand coffin like Lee plus he can't stay in gate modes too long since it rips his muscles apart.
Tsunade will tank all of Gai's hits and heal herself with her Yin seal, he'll get exhausted from using the gates and she'll 1 hit him.
Mei will surround herself with lava and boiling mist so he can't get near her without melting. Combine hidden mist to weaken Gai's sight and boiling mist to melt him as well.
He's low-mid kage level while Tsuade (mid) and Mei (mid-high). Gaara high, A High and Ohnoki high