eh, it could also be the case that one of the necessities for advanced civilization to form is for it to incubate on a planet with roughly similar gravity to earth. From there it's just a matter of convergent evolution.

The gem's physical forms are described as light with mass somehow. I'm sure if the gems are able to induce mass in light, then they're also able to modulate density through the same process that gives light mass in the first place.

I dunno. I mean I kinda have a problem with that rationality. I mean, Finn's had a love interest in two different characters now, Jake is the father of children with his love interest, and there's entire plots about romances, so why does it suddenly become out of place with it's a queer one?