FSR requires DirectX 11, 12, or Vulcan; it's even made pretty clear on GPUOpen that those are what it supports. It's not impossible a player would hack together a version that worked with DX9 in some way, like dx912pxy, but it would be unsupported in the same way.
No developer is going to try and cram it into their older DX9 game via open license when it's not officially supported, and there's no reason for AMD to try and make it work with DX9 themselves, so it's not going to happen. ArenaNet aren't going to use the open license to try and make FSR work in GW2 in any official capacity, even if it were technically possible.