German 800 mm guns would be exposed near Gibraltar

How effective would Germany's 800 mm railway guns have been against Gibraltar?

Not very effective; the few viable firing positions would be easy to identify and attack by counter-battery fire, aircraft, or commandos, while using Spain would also carry major political costs.

The weapon's range and shell weight do not solve the problem of moving and emplacing it within reach of Gibraltar. Limited terrain would reveal likely positions, allowing British guns and aircraft to concentrate on them, while deployment through Spain would risk forcing Franco into a war he sought to avoid.