When you use Gatekeepers, you have to specify "session target ras" to tell it to use RAS rather than specifying the IP address of the remote gateway via "session target ipv4:x.x.x.x".
Do you issue the "session target ras" command in the dial peer on the Gateways, Gatekeepers, or both?
Dial peer says - session target RAS.
Gatekeeper has zones and prefixes.
This allows for the gatekeeper to point calls back and forth between different zones in the 'gatekeeper' configuration mode.
The gateways just need a single dial peer that says 'If I don't know where to go, ask the gatekeeper'.
Obviously fairly broad and this isn't always the intention, but it's the gist.
For more details you can pick up the gateway/gatekeeper book. It's fairly good.