Welcome to the forum. That’s a great print, and part of the fun of collecting is piecing the history together.
On the site here we put had put together a “facebook”-like view of many of the officers we could find (American, British, French, Spanish). I checked out the British officers here: http://revwartalk.com/People/british-military-personnel.html .
There was one individual whom was a Major-General and has a similar uniform, though the face isn’t a perfect match. His name is John Moore: http://revwartalk.com/British-Military-Officers-and-Personnel-J-Z/john-moore.html . In your image, I don’t recognize the face myself as someone familiar to me.
In some internet searching, I came across a site that talks about British Royal Marine uniforms, citing some modern-day film productions showing examples of the uniforms. About mid-way down this page a man is wearing one that looks similar to the one in your print : http://manthecapstan.wordpress.com/2009/01/16/behind-the-uniform-addendum/ . If he might be a Royal Marine, it would be very possible he served at Gibraltar. Also, you might have already come across this in your research.
I wish I recognized his face right off the bat! Maybe some other folks will come along whom can share some other insight, it would be great to solve the mystery.