It would never happen, the Spanish would have to find serious allied forces, which would be difficult due to Britain's presence in the political world, even if Spain could muster worthwhile allies, Britain would almost instantaneously find equal allies.

The other major favour for Britain is their strong naval force, which would be very relevant in a war over Gibraltar for obvious reasons, while Spain does have a decent naval force, it's terrifically outmatched by that of Britain. Britain's naval auxiliary ships alone weigh (by displacement) more than the entire Spanish Navy, that's not including any of Britain's primary naval vessels. Hopelessly outmatched is probably the best way to describe Spain's chances.

It also has to be said though that this happens everytime there's a governmental change, Spain raises the issue with every reelection in Britain, as many other people have said, it's usually just a way for the Spanish government to distract from their corruption.

TL;DR, Spain are just using this as a distraction from their corruption as they know they stand 0% chance in a war against Britain.