Follow these 5 steps. Good luck

Step 1 Set up a GIzmo5 account.

Step 2 At Gizmo5's home page, upper right area, click a green button with "Download for desktop" text inside it.

Step 3 When download finished. Install it. This is the soft phone interface of Gizmo5.

Step 4 Add the "Gizmo5" phone number to Google Voice. Same as any other phone, just be sure to select the Gizmo TYPE

Step 5 Immediately after adding the number in Google Voice, it will call your Gizmo5 number for verification, by asking you to input a two digit number on your softphone's dial pad. Meanwhile, your Gizmo5 softphone should ring, click accept call and input the two digit number that Google voice required.

Now, all calls to your Google Voice number would ring your Gizmo5's softphone.