It's not because you are using BA holidays, its a booking problem full stop at present! [I.e. you should get the same error when doing a simple flight booking.]

Not possible to book ANYTHING connecting TO GIB at present, think the problem is how the fares have been loaded, mentioned this to Dilly by PM a few weeks ago, meant to get back to him with the details.

Outbound from Gib isn't a problem, but the return fails for the same reason.