These characters are some of the most thought provoking in T.G. literature and each story is unique in its setting and style. Each story has its merits and fan base in T.G. literature.

Tuck/Valerie is most definitely inter-sexed. Tuck looks like a boy, but he has female reproductive organs and is developing as a girl now. Rachel Greenham has posted a fan-fiction [Tuck Squared.] Her story takes the Saga Of Tuck in a new direction and her next story [ The Taken continues the story.

Drew/Gaby according to Maddy Bell is not inter-sexed. Drew's body has yet to start puberty yet. The entire premise is that Drew gets mistaken for a girl and had Gaby moments when he dresses as a girl thanks to events that he can not control.

There are different fan-fiction stories that have Drew turning into Gaby so there is no need for Maddy to do it.
Mark/Marcie is a T.G. girl who found her true self thanks to events and is now happily Marcie. If she in time does prove to be inter-sexed is yet to be determined. But whichever way it goes, I hope that Marcie stays the same way and has many misadventures.