Is It GRE? GRE Protocol Encapsulation?

If you don't have exactly the same hardware as a router on both sides, it just won't work no matter how theoretically correct you're trying to set up the configuration in those routing devices. It is possible to do it with OMA VPN, but you have to specifically opt for the Ethernet style interface rather than just the IP tunnel style interface. The other thing I'll mention is if that is still an approach that you find very interesting, neither wire guard nor open VPN should be your first go to. If you want to look at the idea of setting up and everybody's on the VPN style network, I would highly recommend investigating Nebula.