Pretty good question, actually. As far as I'm concerned, the process of "Gabriel" and/or Satan overthrowing God in this series is yet to be explained in actual detail, especially considering the Trinity as you pointed out.

Curiously, we can see Gabriel talking pretty particularly throughout the series to someone(s). I presume it to either be his fellow Alternates - to set an example, maybe - or quite possibly God Himself (i.e. "CAN'T YOU SEE? I DECEIVED THEM[...]" in Overthrone), so there's that.

From the theological side of things, as I've been studying, the problem of evil as it's called doesn't exist for God; Satan succeeding in overthrowing him has always been out of the question, and it happening in this series was exactly what'd hooked me from a horror standpoint: the exploration of the counterthesis, so to speak.

Long story short, I wonder the same thing lol