Issue
The GDM-8341 DMM is not recognized as a VCP when you plug it into the raspberry pi, which means you can't use pyserial or pyvisa to talk to the instrument.
Solution
This problem was really frustrating, but this post here sent me on the right path: https://www.raspberrypi.org/forums/viewtopic.php?t=219232
sudo modprobe cp210x
sudo sh -c 'echo 2184 0030 > /sys/bus/usb-serial/drivers/cp210x/new_id'
(verify your vid/pid match 2184 and 0030 respectively)
After entering that "dmesg | grep tty" shows "usb 1-1.3: cp210x converter now attached to ttyUSB0"
Now pyvisa works.
Hope this saves someone a lot of time.
Answered By - Gardener85