GRE can be implemented between any two routers that have ip reachability between them for IPs used as source and destination of the gre tunnel. One important thing to always remember is that this source / destination pair should never end up being routed over the tunnel itself otherwise the tunnel will be unstable.
In short yes you can use IP Addresses of the subinterfaces as long as they are reachable to the tunnel destination and you make sure that the routing instability does not occur as I have mentioned in the previous paragraph. That being said I do not recommend using physical IP addresses or derivatives of such addresses (sub-interfaces) for use as tunnel source or destination. For your tunnel to be highly available you should try to use loopback addresses as the tunnel source and destination addresses.
