>>281735318
the manga does show that at least up to the elbows she is some kind of mechanical robot housing a bunch of slime which could also mean that her legs up to her thighs and her hair are openings letting slime out, also Maddo said he was kinda hasty when making her with spare parts and does mention wanting to make a geldroid 2.0 when he thought Gelee was getting out of control so my guess is that Gelee was just a prototype thrown together to see if he could use Puniru's slime essence to create an inteligent robot and use Gelee as a way to further his research about adapting Puniru's slime abilities into a robot and get funding to show it is possible but since Gelee became basically a living being he cant just get rid of her or control her fully.

As for why she is a girl the manga does says that Maddo thinks Puniru has a kind of "metaphysical core" so maybe he thought that making a body that looked like Puniru would be the easiest way for Puniru's essence to adapt to it.