How to remove rust in a gate?

Clean off the loose rust and paint with a wire brush. If the rust is fairly new, you may be able to use a power washer to loosen up the flakes of rust and paint.
Use a cleaner/degreaser like the Grease Doctor. You want to be sure the surface is free of dirt and any oil and grime that may be present. Thoroughly rinse the surface with water.
Once dry, apply the Rust Doctor. A trick to make painting the rails and decorative surfaces easier is to use a painter’s mitt. Painting intricate pickets or rails can be difficult, so using the mitt makes the job go faster. The rusted areas will turn black. It is best to apply the Rust Doctor to the whole surface of the project. When Rust Doctor is applied, it also leaves a clear primer on the surface. You can decide to leave it black, or paint the surface with any paint type or color.

Try using this

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Try using this
https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=rust+doctor&_encoding=UTF8&tag=globalventu04-20&linkCode=ur2&linkId=f19f6fa2678d5526dafe436d2924705f&camp=1789&creative=9325

http://www.wikihow.com/Remove-Rust-from-Metal

http://www.wikihow.com/Remove-Rust-from-Metal