Geb honestly gets the short stick in a lot of the story. Not specifically in the comic, but his entire lore. Life just sucks a lot for the twins and I can't even elaborate fully why but it's hinted here and there that if nothing else, their identities, particularly Geb's, is often subject to abuse. It leaves marks, scars, traumas et al.

Needless to say throughout Geb's life he feels like an outcast, a monster, the "lesser twin" that everyone just tolerates to get closer to his sister. He's the weird one, the one who can't do anything right, the one who should be more like *her*, more confident, more active, more sexy, more everything. He's always trying to mimic her, be her, terrified of losing her.

All "identical" twins reach a point in their lives where they develop their own personalities and go separate ways in life, even if they remain close as siblings. It's usually in puberty. If it wasn't obvious by now, for Geb and Nut they still play the identical twin thing even in their 20's (where they are in the comic). By the time the (planned) sequel happens, this has changed and Geb is exploring being different to his Sister, seemingly exhausted by all the parties and sex and hedonism, taking on a more "normal" aesthetic and just chilling out with Toto elsewhere. After all, he was always the "quiet" one, the one who seemed more sensible and rational of the two.

But normal? No. He was always... drawn to darker things, morbid things, monstrous things. Sure they filled him with terror but also something else. A sense of relatability he couldn't quite allow himself to feel. Even though he dates a mimic, even helping said mimic raise children, even though staring into the existential dread of the void of snarling maws and billions of blinking eyes made him feel a weird sense of longing at the same time of wanting to run very, very far away.

Needless to say eventually it was going to crack, and with the help and support of said mimic partner, comes to just develop his own style and aesthetic, to appear more monsterous and horrific as a way to present more how he felt internally. Not as a form of self hate, but as a form of empowerment.
He's a freak, a monster. And that's just fine to be~