Sorry I thought you already had Gizmo set up. You have to have the Gizmo client installed and running on your Nokia tablet. After you have registered an account and logged in, go to Menu>Account>My Profile and scroll down to SIP number and you'll see sip:1747ABCDEFG@proxy01.sipphone.com and it's the 747ABCDEFG part that you need to enter as a phone (and type is Gizmo) into Google Voice. Then you answer the call in Gizmo (accept) and find the Dialpad tab to enter the 2 digit number.

I'm going to get around to writing up a tutorial for...non-US and US-based with no physical phone people soon.