To try and answer the question directly...best place to be is to be sitting window seat ride hand side of the aircraft. Generally breaks through the Spanish coast then over water and around the rock - therefore Gibraltar always visible out the right side windows. If landing is in the other direction it does not really matter where you sit as decent and landing is with water only visible until over the threshold. It can also be quite nice taking photos of the rock from the right hand side as the aircraft makes it was around the rock for landing....and if you remain observant enough you will see the runway. Happy trip and keep the seat belt tight!