A Gas Dock is, unsurprisingly enough, a dock where gasoline is sold. Pumps are generally mounted to the dock itself, rather than being on shore. Gas docks are generally attached to some sort of resort, marina, or harbor.
The gas sold at these establishments usually comes in two flavors: Regular unleaded, and a 50:1 pre-mix for two-strokes. Some two-stroke engines use a 25:1 gas to oil mixture (or other ratios), so almost without exception they also sell containers of the pre-mix oil seperately, as well.
Prices for fuel are inflated at gas docks, with regular unleaded gasoline generally costing from 25 to 100 percent more than land-based gas stations in the same vicinity.