To anyone who didn't expect this... were you even paying attention at all?
Characters die. The doppelganger gains new forms. That is how this manga works.
Gugu had gained the ability to be a human flamethrower... also muscles. It was fucking IMPOSSIBLE for the manga not to want to give that to the freaky doppelganger protagonist, ergo Gugu's death was a foregone conclusion.
Why are people even acting as though it was surprising in any way, shape or form?
The little girl's death in the previous arc was a little surprising for the reason that it wasn't functional and didn't really contribute anything of worth. It was a meaningless death. But this one... that kid was just a walking bundle of death-flags all along.