I've been struggling with publishing RDP on my test UAG appliance, as it was not the same as IAG.
First of all, publish the application which is located on the "Terminal Services (TS)/Remote Desktop Services (RDS)", where you have "RemoteApp" for use with Windows 2008 Terminal Services, Remote Desktop (Predefined and User defined), as well as the two options available from IAG.
I can't use the two options from IAG, as they do not support 64-bit clients (and I use Windows 7 64-bit).
Publish the application as Remote Desktop, either predefined so no user interaction, or user defined where the user will need to specify the desktop to connect to.
Give the application a name, set up the appropriate access policy, define the server to connect to, configure the client settings, ensure the portal link information is correct and ensure the authorisation is correct.
As I found out, this alone will not work and your will receive an error, which points to certificate issues.
As the UAG appliance is the Remote Desktop Gateway, a certificate will need to be applied to it. Go to the Start menu, and select "Administrative Tools/Remote Desktop Services/Remote Desktop Gateway Manager". From there expand the server, and you will see an option to apply a certificate to the gateway.
Apply the appropriate certificate and enjoy remote access!