As a Gibraltarian I think this is a fascinating discussion.
An active resistance movement by locals would have sprung up (and I would have been part of it) which could have travelled relatively freely in Spain unhindered by language barriers or foreign appearance. You'd have terrorist incidents across the region which would keep the occupation at the top of the international agenda.
Remember that the 1967 referendum was 12,138 to 44 so opinion on the Rock was (remains!) militant and there would be no collaborators.
Also an invasion would be very difficult. In WW2 Operation Tracer was a plan to leave troops undiscovered inside the Rock indefinitely in the event of an occupation.
I'd suggest that the occupation would be unsustainable militarily without ethnic cleansing of the whole area - and either doing that or withdrawing would in different ways both destroy Franco.