GRE tunnels and how they transmit multicast packets
In many network scenarios you want to configure your network to use GRE tunnels to send Protocol Independent Multicast (PIM) and multicast traffic between routers. Typically, this occurs when the multicast source and receiver are separated by an IP cloud that is not configured for IP multicast routing. In such network scenarios, configuring a tunnel across an IP cloud with PIM-enabled transports multicast packets toward the receiver.
When a multicast IP packet is transmitted over a GRE tunnel, it is encapsulated into a unicast IP packet, with source and destination addresses of the underlay network, thus no multicast mechanisms are needed on Layer 2 for the GRE network segment.
DMVPN is based on GRE so it too supports multicast in the same fashion.
Broadcast traffic is also sent through a GRE tunnel in a similar manner.
Links:
https://www.cisco.com/c/en/us/support/docs/ip/ip-multicast/43584-mcast-over-gre.html
https://forum.networklessons.com/t/eigrp-passive-interface/1243/15?u=lagapides