The tunnel?
Even when and IF the tunnel is built you will still have to walk across the runway as there will be no pedestrian access through the tunnel. Nothing to do with extra services at all - please do not forget that it is a military airfield (RAF North Gibraltar) and used by civilian aircraft at the behest of the military. If there was a serious flare-up in the area (i.e. North Africa, etc.) and the airfield had to be used as a staging post etc. civilian flights would be cancelled.
Where any problems arise is at the border with Spain but with a new Mayor of La Linea just recently elected who is more sympathetic and wants more co-operation with Gibraltar local difficulties should be ironed out more quickly.