I copied over my configuration from Trixbox to my new PBX in a Flash install. I could make outgoing calls over Gizmo5, but incoming calls were disappearing. I found this post on the Gizmo5 Forums that cleared things up.
change the context=* field to: context=from-pstn. I reloaded asterisk and bam, everything worked. I had copied that field directly from Trixbox which has a Gizmo5 module. PBX in a Flash needs to have that setting manually configured.