Hello. My name is***** can assist you with your question.
There are detailed steps on how to do this on the following page:
Please a me know if this answers your question or if you need additional information.
That page explains exactly how to do it. The routing that you set up with the tunnel is what takes care of directing traffic to the tunnel. If the router recognizes the destination network as being on the other side of the tunnel it will use the tunnel as the gateway. What do you seem to be missing from that page? It does give you a very explicit step-by-step for creating the GRE tunnel