I'm probably a little confused.
What is the source ip address and destination ip address of the ping attempt?
Typically these would be interfaces outside the tunnel interfaces themselves:
port --- tunnel --- port
The source address of the ping needs to have the return route.
If you are going to doi GRE over IPSEC then these are separate ip addresses and ranges. This would be the interface ip address routed down the tunnel.
then the GRE interface will have routes for the LAN subnets on the other side and pointed to the GRE interface.
http://kb.juniper.net/InfoCenter/index?page=content&id=KB19372
In any case you have a static route into the tunnel for the ip address behind the tunnel interface getting the traffic.