Thank you , I'm givin it a try ....
Good luck - oh, and re-reading my last post's advice, I realized there was one thing that was ambiguous:
If a customer DOES have a 192.168.100.x network and you can't convince them to re-number, then you'll need to put a masquerade rule on the office router in their GRE interface:
chain=srcnat out-interface=gre3-C action=masquerade.
I meant on the office router, you'll need to put a masquerade rule on packets going out the GRE interface to this particular customer.
(reason is that without masquerade at the office, the customer LAN would see the source IP address as 192.168.100.x and would consider that to be local, thus the replies would never be sent to the router for transport back to the office. Since the on-site router has 192.168.100.1 in this case, it can't do any masquerading - it would be just as confused as other LAN hosts there - only the office router can tell a convincing lie in this case.)