You can use Gizmo as your SIP provider. My understanding (as a gizmo user, but not having used this heavily yet), is that the Gizmo/GoogleVoice 'integration' is really two different things. For outgoing calls (from the Gozmo softphone), it allows you to call out of Gizmo and show your GV number on callerid; however, the calls are routed through Gizmo's network (SIP to PSTN) and never touch the GV network. For incoming calls (to your GV number), your calls are routed from GV's network directly to the Gizmo SIP number - bypassing the Gizmo SIP/PSTN gateway.
The only limit is for outgoing calls from the Gizmo softphone (that's the 3 minute limit). Incoming calls (via the GV gateway), are unlimited. So as long as you place calls using the GV webinterface, GV will call your Gizmo number and the call time will be unlimited.
As an aside, it's possible to accept and send calls on Android using GV/Gizmo, using (as I understand it) an app that manages initiating the call from GV so the outgoing call is pretty seamless. I'd assume similar devices could be configured similarly.
Of course what would be great is a SIP number for your GV account.
For now, what you want is certainly possible, just might take either a bit of hacking, or accepting that any outgoing calls have to be done via GV's web interface.
UPDATE: Here's what looks to be (I haven't tried it myself) a way to get a mostly transparent outgoing calling from a softphone or hardphone. You sent up a proxy for outgoing calls, which in turn calls GV via Gizmo, and initiates a call back sequence. So you dial, get a busy signal, hangup and answer when it rings. Not perfect, but it would allow you to use a hardware or software phone with out needing to use the GV web interface. This will likely work with your SPA 210, as long as it supports the required proxy settings, and you run the proxy application on an accessible computer.