The game is simple but funny, the levels are good and well thought. There seem to be many, it's cool !
Okayyy, took me a while to figure out how everything worked. The onboarding is a bit difficult, and lots of player don't bother with reading the controls and expect everything to work immediatly. I think there's some work to do on that.
It might seem stupid but I was playing in the browser and with the itch.io page background of your game, I couldn't figure out in the first level that there was a blue door (didn't notice it as it seemed part of the background). Of course this could be improved by changing the graphics of your game, maybe using some premade tileset to give it all some personality, I'm sure you could find pretty stuff on itch.io or in the asset store. But if you wanna go quick, maybe juste change the background of your itch.io page to avoid the confusion. (and having a web version is super cool, thanks a lot)
For the controls I saw many differents things indicated, it's not the same thing said on the itch.io page, in the explanation when you open your game and in the explanation when you lose. This is all a bit confusing.
I think that you should be able to use the left click to start the level too (not just change the gravity), it's kinda of a bother to have to click enter only for that. And with the right click allowing the player to restart the game, the whole game would be playable with the mouse, which is more comfortable.
Simple additions to make it better are a main menu, and a menu with the list of levels to be able to come back to ones you have already done and stuff. And as I was saying before, graphics, this would help your game a loooot honestly
But the gameplay is really cool, this is good for a first jam, well done to you !