WireGuard 'clients' are already statically assigned IPs. It seems like you're looking to either update your LAN-side routing tables but it might just be far easier for long term use to set up a Site-to-Site configuration.
Note WireGuard is technically a peer-to-peer technology; 'client/server' references are used as holdover convention/terminology. It's the routing tables that make the difference.
You haven't stated what is your GL.iNet device but S2S is supported, IIRC.