Follow this guide but don't use sipdroid:
Never Run Out of Minutes Again: Make Free VoIP Calls on Android Phones - How-To Geek
Install csipsimple instead from the market.
Add your sipgate account to csipsimple.
Then Settings -> Filters -> Click your sipfate account -> Add filter
Auto Answer
Contains
Then write in your google voice number.
If you use sipdroid, you'll have to answer a call every time you want to make a call.
When you are done with the guide, log into your Sipgate account at sipgate.com, change your voicemail setting to answer after 60 seconds so that your google voicemail will pick up first. You'll also want to delete the "When I am on the phone" and "When phone is offline/unavailable." rules. Don't set those up to forward to your cellphone because they charge you mins for those forwarded calls.
Instead, set google voice to call both sipgate and your cell phone at the same time.
When you make calls, google callback will make your voice account call csipcimple through sipgate, and csipsimple will autoanswer and your outgoing call will start.
When you get incoming google voice calls, they get routed to your sipgate account which csipsimple is registered to. You'll also get the phone call from google voice on your cell phone line so that if sipgate doesn't work, you'll still get your phone calls.
Only give out your google voice number and your friends will only get calls from your google voice number.