Hello Trippi,

the tunnel source and tunnel destination has to be addresses handled by GETVPN, ideally you should use loopback addresses on two routers.

on the tunnel you assign an IP address taken from a /30 for which under OSPF you have a network area 0 command

A single GRE tunnel cannot be enough you should use two tunnels between two different pairs of routers using loopback addresses as source and destination.

It is important that tunnel source and destination are not advertised over the tunnel itself or you have recursion and the tunnel and the OSPF adjacency will flap.

Hope to help

Giuseppe