There have been threads about this in the past. I recall something about mariners having problems with Visas and there are solutions.

In covid times there were months when Gibraltar was a green country and Spain amber, resulting in anyone diverted having to isolate on return to the UK and also some unscrupulous people heading for Spain would deliberately travel via GIB in order to avoid being detected as having visited Spain. I think it ended up with the land border being shut at one point.