Sipbroker used to work quite well with Gizmo, but not lately. Not since late last year, at least. Since then, about half my calls through Sipbroker to Gizmo disconnect immediately as you describe, or result in a busy signal. Doesn't seem to matter which Sipbroker PSTN access number I dial into.
It's worse when calling into Gizmo's conference rooms (1-222-xxx-xxxx). In dozens of attempts over the past few months, all have failed in the same way.
Both used to work. I don't know why the service has degraded. I've also reported this on the Gizmo forums, but have received no response there.
Although there aren't as many of them as Sipbroker PSTN gateways, you might try using
PointOne PSTN gateways instead. They always work for me.