I work on restoration projects where epoxy has been used over wood that gets wet, like historic fence restorations.
The wood moves,and the plastic epoxy doesn't. Water gets under the plastic. The wood discolors and rots. The plastic comes off in sheets in some places, in others it sticks.

Build a traditional gate, through mortise and tenon, pegged and wedged. Everything will be flexible enough to handle the normal wood movement of a gate out in the weather. There will be wood movement, the wood will weather. It's a garden gate, not a library cabinet.