AT&T U-verse

You really can't get rid of their gateway[*]. It's required to authenticate access to their network. The closest you can get is to put your router behind the gateway and use the IP Passthrough or DMZPlus feature. It's not a true bridge mode, but it does bypass NAT, which is really the important part.

[*] People have been able to hack their way around this. One involves a technique where the gateway is used to authenticate access, then "removed" from the network. This lasts a few weeks or until there is a power outage. The other technique involves downloading the security certificates needed to authenticate your own router. Both of these techniques are not straightforward, so if you want something simple, go for IP Passthrough/DMZ Plus.