I’ve never ‘mixed moded’ OVPN & WG Clients so take this all with a box of salt as the coffee hasn’t fully kicked in yet:
Provided the GL GUI allows both active OVPN & WG Clients, OP would have to test Customize Routing Rules for GL GUI Proxy Mode/VPN Policies, it seems. I’ve never used it. Auto Detect policy could work if WG Client/Server confs are set for proper AllowIPs routing to isolate WG only for those specific LAN Clients.
This would override all other VPN Policies/Proxy Mode(s), of course.
Again, this is all theoretical using stock firmware. Custom iptable rules may/may not be required.
(But I still prefer PBR ATM or GFTO. /s)