We have several clients that are looking to expand their use of GTM to more 'produced' virtual meetings that require external camera or switched program feeds as live sources in the meeting. I don't see current solutions/answers in this board specifying which (if any) encoders or capture cards are supported by GoToMeeting.
In most cases we would use a USB3 or Thunderbolt-connected device from Matrox or BMD, but obviously the drivers for those devices need to be recognized by the GTM application as a 'camera' or accepted input source. What supported devices can currently be used for external 720p or 1080i live video inputs?
What GoToMeeting is looking for is a basic plug-and-play webcam device that works through USB connections and does not need its own software to manage. In your case it doesn't sound like that setup would be compatible.