Well, I finally got Gizmo Project working on my N800. The solution was to NOT use the Gizmo Project Client that I downloaded - instead I configured the N800's built in phone. You'll need your Gizmo Project phone number (Start Gizmo Project and click on settings - it is grayed out) and Gizmo Project Password to do this.

Click on Settings
Click on Control Panel
Click on Accounts
Click New
Click Next
For Service: Select SIP
Click Next
User Name is your Gizmo Phone Number (ie 1747*******)
@ is: proxy01.sipphone.com
Password is: your Gizmo Project password
Click Advanced
In the Connection tab -> Outgoing proxy is: proxy01.siphphone.com
Port is: 5060
In the STUN tab -> Uncheck Auto-detect Stun
Stun Server is: stun01.sipphone.com
Stun Port is: 3478
Click OK
Click Next
Account Name: leave blank
Click Next
Click Finished

Now when you want to make a call using Gizmo Project you just click on the contacts icon (below the globe on the home page) and click on "new internet call".

Internet call will load up and be idle. You can make a call by clicking on the numeric keypad on the left and filling in the number you want to call. Of course you are using Gizmo Project so they'll charge you their normal rates. You can also use Grand Central Mobile at www.grandcentral.com/mobile and make your calls for free.