Put on work gloves and protective eye goggles. Place a high-quality wire brush attachment on the drill, and work it over all of the rusty areas on the metal gate.
The wire attachment should strip all of the rust off the gate. Be sure to work between the bars and into all nooks and crannies until the rust is removed.
This can be a tedious process.
Sand the surfaces of the gate, paying special attention to the areas where you removed the rust. Use a rough sandpaper, in the 60-grit range.
Sand until all the surfaces are smooth.
Mix a strong detergent-type cleaner with water in a bucket. Dip a scrub brush in the solution and wash the entire gate.
Rinse with water from a hose with a spray nozzle. Allow the gate to fully dry.
Prime the gate with a rust-inhibiting primer designed for outdoor metal surfaces. You can use either a spray or brush-on primer.
Follow package instructions for the proper method of application and appropriate drying time.
Spray on an exterior enamel of your choice. Again, follow package instructions, but in general, hold the can about six to eight inches away from the surface and spray in long, smooth strokes, making sure not to apply it so heavily that it runs or drips.
Wait for the first coat to dry, then apply a second coat for superior protection.