On beds I build for contractors, farmers etc a 1 ton or larger truck I use 1/8" (.125) hot rolled floor plate which weighs 6.16 lbs per sq ft. If you looking to save some weight and will not be hauling heavy equipment with weight concentrated in small places such as tires on a skid steer you could go with
12 gauge @ 5.25 lbs per sq ft
14 gauge @ 3.75 lbs per sq ft.
(Floor plate thickness is measured WITH OUT the hump)
Floor plate is not necessary for a truck bed but it look better and usually will stay painted longer.
Smooth hot rolled 11 gauge (.1196)is just under 1/8" is 5 lbs per sq ft.