Really cool puzzle concept, and great execution! Loved playing through this and not only making my way through the levels, but learning how to utilize the gravity mechanic to its fullest. Speaking of that, though, and maybe you think I'm an idiot for this, but man did I get stuck on level 6 for an embarrassingly long time, haha! I felt like this level should be moved to much later on, as the gravity slingshot technique it teaches you felt way too challenging and unnecessary to learn at the time, considering the much simpler levels that followed it with more standard box stacking mechanics and such. Speaking of that, it's kind of a double-edged thing: I thought the gravity slingshot level was a bit too complex and early, but that's only because the majority of the levels didn't rely on tricks like that and instead were more basic, but perhaps that's just a sign that the levels should've used gravity in more interesting ways instead to spice it up? Felt like it did have a good variety of mechanics though, just maybe the difficulty curve was a bit wonky at times.
In terms of controls, maybe it's just me, but I felt like the alternative control scheme (always left/right and up is always jump) made a lot more sense than the default controls. I feel like the controls should be switched, with the alternate being the default. Maybe you think that's silly, since both are available options, so why does it matter which is default, but I do feel like a dev needs to set forward a good default control scheme for a first impression and intended experience, as many players in my experience will not bother trying to try other controls schemes than the default: they'll just complain and move on to another game.
Anyway, glad to see another cool puzzler from you!