I have had a couple of GMC-500+ units, and I have a 600+ on order. To use these more easily with applications in .NET, I wrote a simple couple of classes to control the devices over USB / UART, and is based off of commands found in RFC1801.
This is an entirely open-source (GPLv3) project, so if it's of use to anyone, I thought this would be the place to make it available.
It's on both NuGet and GitHub for ease of access and visibility into the source code.