Explanation:
The GRE keys must match.
To separate the traffic between the two virtualized networks, the GRE headers on the tunneled packets include a GRE Key that provides a unique Virtual Subnet ID for each virtualized network.
References:
https://blogs.technet.microsoft.com/keithmayer/2012/10/08/step-by-step-hyper-v-network-virtualization-31-days-of-favorite-features-in-winserv-2012-part-8-of-31/