Gibraltar has, in the past, played host to A321's and A300's. Both courtesy of the Saudi royal family. C17's and Hercules' were/are also regular visitors, as well as the A400M.
I can't imagine the 738MAX should have too much difficulty... It will be tighter than the A319/A320 and 757, which are best suited to operations out of GIB, but far from impossible I suspect. I doubt Monarch would have ordered them if they had to give up their GIB flights, they're the largest operator!
I may be wrong, but do the MAX's not come with short field package as standard? I know the 739ER does.