There is another preferred way: in the VPN Service config tree there is a GTI Settings part where you can specify the defaults per box. Just set the F18 transport sources to dynamic and at the central box e.g. second IP (if the second server IP is where your VPN Server listens).
Then mark the central box as a hub in GTI Settings.
From now on each time you add a vpn Service to the GTI there is automatically a tunnel created to the hub.
If you need that the branches can communicate with each other via the hub then specify e.g. 10.0.0.0/8 under GTI Networks at the server settings and the real lan networks for the server settings at the branches.