rittf800, when I still had my 770 (thinking that it's dead now still makes me sob) I used Gizmo and it worked best with several accounts I tried. (What was especially worth noting: Gizmo was and still is the one client that almost always gets across NAT-situations in networks I can't control. I still have it installed for this very reason as a backup even though I now use Telepathy-SofiaSIP now on my N800.)

So if you experience problems with your SIP provider and Gizmo, maybe there's still some setting you missed. Did you try Gizmo-specific forums?