It has been. Too lazy to dig up threads, but the general consensus is that it would have been a disaster for Franco (though probably not for Spain, since they'd have been rid of their dictator a few years later), Spain was completely unready for a war so soon after their civil war, they would not have been able to help much or mount a decent offensive against any of their targets (certainly not Gibraltar without extensive German assistance), and in fact, they would have become a liability for the Axis as they would have crumbled very easily in the face of a concerted British offensive and would have needed Germany to patch up their boo-boos as they did with Italy. It is also commonly thought that Franco specifically demanded more and more 'rewards' from Hitler, eventually too much to be possibly granted, so that he would be able to avoid allying with Hitler without openly breaking with him.