It is of course complete nonsense to suggest that Gib is Spanish territory.
As eny fule no it was ceded to UK in perpetuity by the treaty of Utrecht following the War of Spanish Succession, fundamentally a Habsburg claim to the Spanish throne.
It is this clearly and unequivocably British.
The Falklands were never ceded to Britain by international treaty, merely claimed - but equally unequivocably British as no one else had claimed them first. They no more belong to Argentina due to proximity (an utterly spurious and worthless concept) than the Channel Islands belong to France or the Isle of Man belongs to Ireland, or, for that matter that Ireland belongs to Gt. Britain.
If Spain claims Gib it will have to show the Treaty of Utrecht was somehow illegal.
And given their armed occupation of Ceuta and Mellila that is ratified by no treaty I know of this claim is about as hypocritical and irrational as irrational hypocricy gets.