• Why just Gleba?

Why not? Gleba has pentapods, Nauvis have biters, they felt the most natural places to have biolabs.

• But why though?

Because I just wanted to be able to megabase on Gleba, and I felt like that 'needed' the good labs.

• Is this overpowered?

It makes shipping agricultural science notably easier I guess. But importing the other planetary sciences still works the same as ever.