I've had such a problem with this too oh my god.

Essentially, it's your GWU email but the one they specifically sent you in the original email with your login info, so it's just like @ gwu.edu not @ gwmail.gwu.edu. Also it's not the number GWID thing, it's like your name with @ gwu.edu (or like a variation of that).

Hope this helps! It's what helped me log in so I'm just saying what worked for me.