I can go over this from strictly a tactical/operational perspective, there might be someone out there who can go over it more from a historical perspective.

First, as you suggest, control of Gibraltar can confer control of passage between the Mediterranean and the Atlantic. This is important because it allows you to deny passage to both merchant and naval traffic.

The reason why the Rock of Gibraltar (which I assume this question is about) is so important is really just a matter of geography. Any of the land on either side of the Strait would be perfectly fine for establishing a coastal defense fort from which to attack and harass ships from. Common coastal defense guns from around the turn of the 20th century (like the BL 9.2 inch, BL 15 inch, BL 6 inch MK XIX, 5"/51 caliber gun, the 10-inch M1895, and the 12-inch M1895) could easily reach out across the Strait, and even lighter guns had enough range to deny the deepest part of the Strait to enemy traffic.

However, even if all of the land on either side of the Strait is basically equal for attacking and harassing ships, not all land is equally defensible. Just plopping down some coastal defense batteries along the Moroccan and Spanish coasts would be more or less fine for area denial, but they would also be fairly easy to neutralize. All you would have to do is land some troops in an area where the guns can't touch the boats, and then march them overland to silence the batteries.

Just look at a map or a satellite image of the Rock of Gibraltar, and you should get a sense for just how defensible it is. It's a rock jutting straight out of the ocean. It has commanding views and fields of fire on all four sides. Three sides would be extremely difficult to approach except for with assault landings (which weren't really a thing until World War II), and even then, you are disembarking into cliffs for the most part.

The easiest way to approach Gibraltar from land is by coming along the isthmus connecting it to the Spanish mainland. Even this would not be an easy option by any means. Charging across a narrow, open piece of land with little to no cover into prepared defenses and artillery positions that likely have the entire area pre-sighted would be a recipe for disaster.

The advantage of the Rock of Gibraltar is that you can both deny your enemy the sea, and defend your own emplacements with a far smaller force. The Rock itself acts as a force multiplier. The Rock of Gibraltar allows you to exercise just as much control over sea traffic for a far smaller investment (in both material terms and in manpower) since it is a naturally defensible area.