GIB terminal, as I understand it, now has two entries/exits, one to/from Gibraltar the other to/from Spain.

If you arrive into GIB from, say, LON, and exit the terminal to enter Spain, you are going into Schengen space, and have to clear the same formalities as if you had arrived at, say, MAD. Coming back from Spain into GIB terminal, you are leaving Schengen space, so again the same formalities as if you were flying MAD-LHR.