Owing to the geography and several tunnels not open to pedestrians, there’s not a seamless run around Gibraltar. But there is some very scenic shoreline running in Gibraltar, at the base of and around the perimeter of the rock. Below are some of the best shoreline sections. This map of good cycle roads is helpful.
East Side: Eastern Beach and Sir Herbert Miles Rd. Decent sidewalk and pleasant views of the Albora sea from Eastern Beach Rd. south to the entrance of the Dudley Ward Way Tunnel (no pedestrians). 2.4 km, ~100m of elevation. Also, one can run on Eastern Beach when it’s not too crowded, ~0.5 km.
East Side: Europa Advance Rd. from end of tunnel (3rd Europa Advance Battery) to Europa Point). Beautiful views, but narrow road with no or very narrow sidewalk. 1.1 km, plus a bit around Europa Point.
West Side: The best running is the 3+ km between the Ferry Terminal and 100 Ton Gun, using Europort Rd./Queenway / Roasia Rd. South of Camp Point, the road is narrow and interrupted by tunnels. ~3.3 km and mostly flat! Can add a couple of extra side jaunts around the marina.
In Town. Main St. is pleasant for running, as there are lots of shops, restaurants, and things to see. It is mainly pedestrian only and narrow/crowded, so one must go during ‘off-peak’ times or it is not runnable! It’s ~1.2 km between Casemates Square and Horatio Nelson Statue. Good to combine with the west side coastal road of a section of the Nature Reserve.