How is the Spanish claim on Gibraltar weak? They border it, it was taken from them at gunpoint, and it's extremely dependent on Spain for services and labour.
You're misrepresenting the situation. Gibraltar was Spanish for a few centuries ( after the Reconquista), and after it lost a war the UK annexed it. It's not some colony far away, for Spain, it's a part of the Iberian peninsula, and is entirely dependent on Spain.
It's like Hong Kong, in a sense. Land the UK took because it could from a weaker country due to its strategic location, but which is mostly integrated with it's former country in terms of labour and services.
Yes, Spain lost Gibraltar in a war. Pretty much every border was decided by war, so why is that border different? ("we had it a long time" isn't a difference.)
Yes, Gibraltar is dependent on Spain for services. As I pointed out, that's a tool for changing the border. (Blockades are an act of war, so that would be "at gunpoint.")
However, that dependence isn't an argument that the border is "wrong".
Every border is a result of "taken from them at gunpoint." (It's not hard to find folks who had large pieces of Spain taken from them at gunpoint.)
Dependency isn't a claim. It is, however, a tool for taking.