It has been a while but I've used this type of configuration for compatibility and it does work.
In a Juniper to Juniper connection you don't need the GRE tunnel but can use a route based VPN and make those OSPF or other connections directly across the tunnel via the tunnel interfaces themselves.
My recollection is that when connecting to Cisco for these types of peers the GRE encapsulation over the VPN is required for the multicast traffic to work as expected.