Gillian's aged better than David.
She's had some work done.
But why is Mulder still calling her "Scully" in the released clip - her last name? You'd think they'd be on a first name basis by now and after having a child together...
Additionally, you may have heard The Lone Gunman will make an appearance in the series and folks are wondering how they'll explain that one, since they all "died" at the end of their series. So maybe as ghosts, or maybe their deaths were faked, blah, blah, in-universe explanation, blah, blah...
I have no problem just totally ignoring their supposed deaths in The Lone Gunman series. As in, it never happened. Just like Newt, Hicks and Bishop and Alien 3 never happened.
With things like this, you just film a director’s 10-second statement at the beginning of the film saying, “A lot of fans were disappointed when we snuffed your favorite X-Files supporting characters in our Lone Gunmen series. Yeah, we made a mistake. Sorry. As we move forward, it never happened. We hope you enjoy our show.” Don’t even try to do a "Dallas" on it (where the whole thing was a dream).