GRE - Tunnel Addressing

When configuring a GRE tunnel, the IPv4 or IPv6 addresses used on the tunnel interfaces must be in the same subnet or prefix.

By definition, a GRE tunnel will connect two tunnel interfaces on two different routers across a network such as the internet, as if they are directly connected. So by definition, these interfaces must be on the same subnet. In other words, you’re tricking the routers into thinking they’re directly connected. Two directly connected routers must have their interfaces on the same subnet or prefix.