Joining Zentyal GW/VPN to domain/ldap?

[xvdiff]

First post, yay!

I'm currently trying to set up a small business environment using Zentyal. For this, I've installed 2 servers where as one should act as a internal domain controller and LDAP provider, the other one I'd like to use as firewall, ALG and VPN server. Now, after I configured my users and groups on the DC, I would like to join my GW server to the domain, so that domain users will be available in the VPN. However, I have no clue how to do that. As far as I can see, I'm only able to install a directory/domain on the GW or joining it to a existing Microsoft Active Directory. Did I missed something?

Thanks!