The Wall Gate texture shifts when the gate is opened and closed in the Positive Z direction, it resets back when the gate has been opened and closed in the Negative Z direction.

It does not effect the Negative X or Positive X directions.

I only noticed it when making the 32x Fence Gate textures for the John Smith Legacy Resource Pack I look after but it's not the pack as it happens in the vanilla default textures too.

This does no effect the fence gate when connected to anything else only when they are connected to a cobblestone wall (mossy wall also).

Compairing the two model files for the gates (wall & fence) open and closed.

wall_gate_closed.json - fence_gate_closed.json

wall_gate_open.json - fence_gate_open.json

They look fine so it must be something else, I noticed there's no blockstate for wall_gate so it may have something to do with that.