For one of the more easy ways to level a gate at the desired elevation, you'll need 2 clamps ( I Prefer 2 Quick Grips ) and a couple of pieces of lumber 1x2 or 2x4 or whatever is handy. Extend a piece of lumber , on one side , below the gate about the height you would like the gate off of the dirt and clamp it to the gate. When the gate is leveled with your level, allow the second lumber piece to drop to the dirt and clamp that one. Now JustScrewIt.
This is also how a saw horse can be prevented from rocking on an uneven ( 1 clamp and board required ) surface if that bothers you as it does me if I'm zeroing my rifle.