Okay, this might be a bit crazy, but hear me out.
I don't think Gigi is Amy, but I do think she was in the Kappa bathroom that night. We know that she is a former Kappa and suffered from a traumatic event in the '90s. In the last two episodes we find out that she us not only the Hag on Shady Lane but also is commanding at least one of the red devils. The show is certainly set up so that it seems Gigi is Amy, the one who held the baby, the dark-haired Kappa. And most people seem to think she is.
But a few things don't make sense.
I mean, sure, seeing a friend give birth, bleed out, then die from child birth is definitely traumatic. But is it Hag on Shady Lane traumatic? Is it howling and wailing in grief for months traumatic? Is it crafting creepy baby dolls and piling them up around you traumatic? Is it stuck in the '90s for two decades traumatic? I'm not convinced. But you know what would, quite understandably, lead a person to do that? Giving birth to a baby even though you didn't know you were pregnant, bleeding out, losing consciousness, then waking up in a shallow grave that your friends dug. Now THAT is trauma. It's also some serious motivation for revenge on Kappa house.
Here's some of my reasoning, in no particular order.
-Gigi and Sophia have exactly the same hair color, and Gigi even wears her hair the same way as Sophia. They also have similar mannerisms, or so I think with the little bit we've seen of Sophia.
--no one ever actually checked if she was dead. Instead they poked her eye. We don't know how deep they actually buried Sophia. I imagine sorority sisters wouldn't be the best grave diggers though.
-The hang was only seen stealing diapers and milk once, if there was a baby she would either have had the baby a very short time (which is possible) or not at all.
- The hag is heard wailing multiple times, but no one ever heard a baby. This leads me to suspect the baby wasn't ever there. But the Mother was and she was grieving. She had gone crazy, so she had stolen diapers and milk like she had her baby to take care of, but really she was taking care of her baby doll creations.
-This also explains the baby dolls. I'm not sure what happened to Amy would drive her so far off the deep end, but surely if Sophia had crawled out of her grave, and didn't have her baby, putting together baby doll freak shows to keep her company doesn't seem a stretch.
-- this also would be a better explanation of why Gigi is working with the red devil, what better motive for revenge could there be? I also suspect that some of the current Kappa's may be connected to the Kappa's in '95.
--Also, Wes seems connected to that night (playlist, married a kappa) but he doesn't recognize Gigi. Remember, Sophia was a pledge, so he might not have known her, but even if he did, and he believes she's been dead for 20 years, it's hardly surprising that he wouldn't suspect it was the same person.
Of course, for any of this to make sense, I think some one else would have to have known she survived. My bet is on the only one who knew where the grave was, Cathy Munch.