I'm pretty sure that (occasionally) happens outside of doujins. Specifically the (reverse) harem genre tends to have it from what I remember. That or some visual novels use it.
This doujin isn't what I usually think of when hearing "gap moe" though. It's more of Kanan's (perverted) thoughts that something Ruby was intentionally doing.
Oh Yuuki Konno? Yeah she's was (and still is in the games since she doesn't die unlike the in the source material ) one of my favorite SAO character and I really need to see the Mother's Rosario arc.
@random I don't think Dia's a time traveler here. I'm thinking it's just the drawing style that's making Nico and Ruby look younger since Dia still looks about the same age as she does in canon.
Not sure about that. I do know they have a rivalry going on between each other which naturally makes them popular to ship together. Same reason that Asuka and Homura is popular too (probably anyway).