Something I noticed today..... when you deploy a gateway server - the helper object oomads.msi was not copied to the local \AgementManagement directory for agent push. 

This means, that if you have a DC in the untrusted forest, managed by a gateway, that Oomads will not get copied or installed automatically.  You will need to manually copy and install Oomands on any DC's you will monitor in the untrusted forest.