I am using the desktop client 10.5.0 build 16180 with a Plantronics USB478 headset.
The device is fully functional, and I have no issues with other audio applications (eg Audacity, Skype/Lync).
With GoToMeeting, however, the mike gain is permanently so low that I have to practically shout to be heard on a call.
Other potentially relevant information:
Under the GoToMeeting node in HKEY_CURRENT_USER, there are two instances of the device (presumably from the two USB ports I have used it with). One had a DigitalGain key set to 5, the other set to 17. What that means, if anything, is obscure. Changing the values and restarting the application does not seem to affect audio performance in any meaningful way.