An 18-inch coastal gun at Gibraltar

Could an 18-inch gun at Gibraltar reach targets across the strait?

Probably, provided the land mounting allows enough elevation to reach the estimated distance.

A rough map measurement places the proposed targets across the strait at about forty kilometres. The comparison uses an American 18-inch gun credited here with a range of about forty-five kilometres at forty degrees of elevation. A British weapon with similar performance is therefore judged capable of reaching the opposite side. The mounting matters because a shore installation could allow greater elevation than the ship mountings considered. The practical shipping channel and the narrower approaches also lie closer than the full distance to the proposed targets.