As a Regular GIB flyer until Monarch went under I will say that a lot of customers who did use GIB did indeed cross the border to Spain, myself included.
I've heard lots of speculation of this route for the last few Months but I would not be surprised if EasyJet were to start LTN-GIB next summer, plus with the rumours about expanding their MAN base significantly they will possibly increase the frequency from there too.
We shall see I guess, but Jet2 would be a nice addition to GIB.