Yes he's visibly pushing up against it and you can see him at the bottom of that panel under the fortress. The fact he's trying to physically push implies that his air membrane can't hold it by itself, obviously there is no point in him doing that if his Quirk was holding it up by itself.
Also Gentle can't control the things he makes elastic with his mind or something, he isn't telekinetically pushing the air membrane up. Gentle cannot directly attack with his Quirk, his Gently Rebound for example shocked him with how powerful it was when it sent Izuku back.
It's a trampoline, the force that gets sent back is determined by how much force you ram into it. That's why he was shocked, because it showed how strong Izuku was. That's why I find giving a rating for Gentle's Quirk to be silly, similar to Flect I think his rating should be Varies with Elasticity.
Because the power that gets reflected depends on the strength of the attack. Went off topic there...
I don't know how much Gentle is physically contributing in this moment, I wouldn't be surprised if the air membrane is holding most of it back. But I wouldn't be surprised if Gentle shows some more feats in the later chapters, after they stop the U.A. fortress from falling that is.
This is a good durability feat for his air membranes though, since it can withstand this amount of power.