I think GIB will have to be 320, the 321 is too payload restricted for this route. The 757 was ideal for GIB when the demand was higher than the 320. the 321s are economical certainly, but they are under powered and range restricted. Charter routes to Turkey, Egypt well I dont think an all 321 will be a good ide Time will tell when those old faithful 757s are gone, so many routes will have to fall with them, I think.
I go to GIB regularly with MON and it has always been an A320. I know the 757 has done it in the past but i think it was a long time ago since one went to GIB.
Some of the 321's ahve just done Egypt for the winter and by the looks it went well but think they are using the A300 this summer. I also know that EasyJet do the A320 to SSH from MAN so I can't see it as a major problem when the A300 & 757's go.