Oh this is fun. Let me think...
Bobbie would have one relating to his name and his birthday. Something like "bobrod725". (He doesn't have a canon bday but he's a Leo so...just an example lol)
R.J.'s family calls him "Richie", so he'd probably have the word "rich" in his gamertag lol.
I'm not sure what Gabriel's user would be..he's half Korean so maybe something related to that?
David would simply be "davidrodriguez" and some numbers