Glamrock Freddy is Gabriel
(Sorry for any possible spelling mistakes)
Think about it. It makes a lot more sense that he's Gabriel (or at the very least one of the kids from the "disappearences") than Micheal. Because sure, Micheal burned in the fire in FNaF6, an we all know the whole FNaF6 pizzaria being under the Pizzaplex thing, and SUURREEE, Micheal was infused by remnant or something to be able to re-possess his body after he turned into a zombie and he used the remnant to turn into Glamrock Freddy, but I just don't think the whole "Micheal is Freddy" thing is realistic (but when is FNaF ever realistic -.-).
First of all:
Glamrock Freddy acts very childish throughout Security Breach. Of course, he acts as the responsible adult in a lot of the parts, but in some parts, he acts quite like a child. For example:
When you give him Roxy's eyes, he exclaims how amazing the new eyes are in a very childlike manner.
When he tells us how the Cameras work at Monty Golf, he explains it's a very clever marketing strategy, but in truth it's a scam. This is where his childlike naivity comes in.
(There might be a few more examples, but those were all I could find now).
Second of all:
When has Micheal ever been asscociated with a Freddy or a bear from the franchise? It's common knowledge that Gabriel is the one to have possessed Freddy in FNaF1, and Henry was typically asscociated with Fredbear thanks to The Silver Eyes, but Micheal is never the one who has any kind of a relation to a bear/a freddy.
The only time this is true is in FNaF6 when he wears a bear mask to hide is zombified face, but that's a fan interperation and has never been proven canon at all.
Third of all:
Security Breach generally shows a lot of nostaglic elements. The entirety of Rockstar Row is dedicated to old relics from past Fazbear Ent locations, displaying posters from the original Freddy's locations and old robot parts from animatronics the franchise no longer uses.
The blob itself is on big piece of nostalgia, with animatronics from a lot of past FNaF games.
There is even an unused line from Glamrock Freddy where he mentions past iterations of the character Freddy and its past models, specifically how these models did bad things. (Which may be a another piece of evidence. Maybe he remembers doing these bad things...)
That wraps my theory up for now, if I find any more evidence, I will edit this post.
Please put your own opinions, views and theories in the comments! And if I missed anything, definately tell me! Just be nice about it, ok? No hate please.
(Again, sorry for any possible spelling mistakes)