I don't think either GIB route is struggling or will struggle. When Bmibaby offred EMA-GIB apparently it was really popular and the Government of Gibraltar tried to find a replacement carrier to do a route from the Midlands. Both MAN-GIB and BHX-GIB don't operate on the same day either (until May where they will both operate on a Sunday). MAN-GIB has also had its frequency increased and what I have heard is the Monarch flights are always packed and popular. EasyJet's LPL-GIB was dropped because apparently loads were a lot lower than expected and this route had the lowest demand. So I don't really see either of them to be dropped. BHX-GIB has only just started as well so I'm sure over the years it will get even more popular. And GIB also has it's new terminal fully open now so the airport is much better tn it was.
Also a few years ago MAN-GIB was not stopped because of lack of demand, it was stopped because Monarch had issues with the government there at the time.