I was able to successfully connect to the GMC-800 using the Linux terminal interface 'cu'. I used the command:
cu --nostop -l /dev/ttyUSB3 -s 115200 dir
The "--nostop" is needed so binary data coming from the GMC-800 is not interpreted as xon/xoff flow control commands. You might also need to set 'raw' mode using stty.
I used speed 115200, and the commands listed in this forum at:
https://www.gqelectronicsllc.com/forum/topic.asp?TOPIC_ID=10394
I am specifically using "<GETCPM>>". The AT commands did not work for me.