Ran into the same issue yesterday during one of my employer's meetsings. I have an Elgato Cam Link which DOES show up as just a normal webcam to the point that every other application I have used that expects a simple webcam source interfaces with it just fine including Windows 10's own Camera app. However GTM (desktop client) just continued to say 'Failed to Initialize' and had to fall back to my backup C920.
Did some further testing this morning using the test meeting option and it appears GTM doesn't like anything higher than 720p resolution. I have my Cam Link hooked up to a Panasonic Lumix DMC-G85 and usually by default I have it configured for 1080p60 (I even tried 1080p30). Adjusting it back to 720p30 allowed GTM to display a picture from the camera.
The catch here seems to be the Cam Link won't allow the client app to change resolution. It is limited to whatever resolution the attached HDMI device is putting out. So in this case I have to specifically drop down my camera to something GTM will play nice with. So maybe this is good information for others and something to keep in mind.
Thanks for this, it helped me to understand why I was pulling my hair out yesterday trying to join a meeting with my A7iii connected via the ATEM Mini or CamLink 4K.
What I have found though, in case others end up here and are at a loss as to what to do, is that you can feed the camera input into OBS and use the OBS-VirtualCam plugin to output a signal that GoToMeeting will accept. It is messy and clunky but it at least works.
This might not work as easily on MacOS though, I've tried on Windows.
Feedback to GTM team, with the world in its current state and everyone working from home more, it is more common for people to be using higher end equipment from home offices/home studios. You need to support these options or continue to lose ground to competitors. It's sad to say that I can't get my camera to work natively, I can't get my mic to work natively and I can't get my headset to work natively.
We aren't talking space age technology here, just 1080p camera output, an XLR mic with USB DAC and a Wireless headset.
I'm having a related issue where ManyCam shows my video form an external camera, but when I select ManyCam Virtual Webcam in GoToMeeting Setting as my Camera, the video image doesn't come through.
Any ideas as to why that might be and how I can fix it?