That's not strictly true - Franco imposed what the natives used to call the 'last siege' when he closed the border in 1969. This ran until February 1985 ( I know, I was there) when the road gates were re-opened. We were confined to camp on the night for fear of adverse publicity should half a thousand pissed Queensmen appear in front of the World's TV cameras.Even in its most fascist incarnation, Spain left Gibraltar alone. I recall that Hitler wanted Franco to help him take it and the Caudillo effectively told him to do one....
Franco also built an oil refinery just along the coast hoping, it is said, to make the life of the Rock's inhabitants unpleasant with the stench of the smoke & fumes that it would give off. Sadly for Franco, and the local residents, he didn't do a very good weather appreciation and for most of the time the prevailing wind blew a trail of grotty smog across Spain. On the odd occasions that the wind did blow our way, fair play to him, it was fvcking honking.