If I can make this make sense...
Powder coated is a coating and can flake off, causing rust to happen. This isn't
supposed to happen but it does chip and then peel..
It also is more a "surface" application.
Galvanized is a application that goes through & through, so inside and exterior of the gate would be treated....hence not able to rust from the inside out as the actual gate structure is steel tubing.
Steel rusts unless treated with something.
Hope that makes sense.