Some parties have tried to access Gull Rock from Cornakey Farm. There is no parking on the private track to the farm or at the farm itself. Additionally there is no public right of way from the farm to the coast path. Park at either Morwenstow to the south of the crag or Welcombe Mouth to the north and follow the South West Coast Path to the crag.
Actually, neither of these options is to be recommended unless you fancy an overlong walk.
Your best bet is to park considerately on the side of the road either at SS 2215 1759 near Mead or SS 2175 1692 near Marsland Manor (space for 2 or 3 cars at each). From both spots straightforward paths lead down to the beach at SS 2119 1742 (Marsland Mouth), from here boulder hop southwards to the crag.