Hello Deep
First of all, one major difference between unicast and multicast routing is that for unicast routing, we only care about the destination and how to get there. For multicast routing, we care about the source as well. PIM uses the unicast routing table to check what interface will be used to reach the source. More on this can be found at this lesson.
Now to your question. If a GRE tunnel is used only for multicast traffic, then there will be no unicast routing
information concerning routes over the GRE tunnel. Therefore you will run into the RPF problem. If you do use unicast over the GRE tunnel, unicast routes will be available and you will not have the RPF problem.
I hope this has been helpful!
Laz