Which is why the sensible course is to invest it with a 2nd line division.
I dont think you can take it with one anyway, the level of casualities would be such as to render it non-operational before you succeeded, I think youd need at least 2. And an enough artillery to show you were actualy doing something, otherwise even german troops morale is going to be in the cellar.
In OTL the Germans par say didn't plan to use an actual single division anyway; it was supposed to be Grossdeutchland regiment, 2 mountain regiments a Jaeger regiment plus the brandenbergers and maybe a few spanish troops to take the coaling island and whatever (plus a panzer corps at seville in reserve to contest possible counter landings)
given the narrowness of the front, the Germans would find it hard to have more than 6 or 7 battalions at the front at any one time anyway
the artillery and airpower would be the great leveler; 26 battalions of artillery plus hundreds of aircraft based out of Seville and San Rourque capable of flying 5x sorties per day; the British would lose all of their surface positions almost immediately due to inability to operate in daylight with stukas prowling over their heads; its a question of how long does it take the axis to take the inner workings of the rock... me thinks they make it to the water casemate areas with a decent amount of casualties; blow them up and then render british resistance a diminishing factor