They have a territorial claim over the rock so they may well seek to advance that objective within the framework of brexit negotiations but, as noted, the border you forecast already exists as Gibraltar is a duty free tax haven, outside of the Schengen area. In the context of such debate other member States would be likely to question continuation of special duty free status for certain Spanish territories i.e the Canary Islands. However, the Spanish could argue that current inhabitants could easily be accommodated within the United Kingdom of Great Britain and Northern Ireland by tuning (some of) "the six counties" into a duty free monkey sanctuary with comparable taxation advantages.