Hello Narad
GRE and DMVPN support multicast, but it is not implemented in the same way as in a simple network without tunneling. A multicast packet that is tunnelled within a GRE tunnel, will maintain a multicast destination IP address in its IP header, but this is then encapsulated into a second IP packet, which is the GRE packet encapsulation, in which a unicast IP address is placed in the destination field of the IP header.
So a multicast layer 3 packet is encapsulated (tunnelled) into a unicast layer 3 GRE packet. When multicast packets traverse such a tunnel, multicast MAC addresses play no role in the multicast process.
I hope this has been helpful!
Laz