I have a couple of routers plugged into my U-Verse RG, and it works great. One is for Wireless-N capabilities and the other is just for some extra ports at one of my TVs (using an old Linksys router as a switch for a slingbox connection).
There are two ways you can go. One, use the RG as a "modem" and have your router (the wireless-N router) manage the network.
The second method is letting the RG manage the network and use your wireless-N router as a bridge and wireless access point.
I am using the second method. To do this, connect a LAN port on your U-Verse RG to a LAN (not the WAN) port on your new router. The RG is managing the network and your Wireless-N router is basically acting a switch (and wireless access point).
As the others have said, Disable DHCP on the Wireless-N router and turn off the wireless radio on the RG.
AT&T U-Verse Router guide
(Links at the bottom of the page for each method)
Setting up a Tomato router behind an AT&T 2Wire UVerse Router