This USB-to-Serial device is recognized by the kernel since very long. So, unless you use a very strange Linux distribution, the device should be available right after connection without any further installation.
Do these steps:
Connect the GMC-300E and type "dmesg" in a terminal window. At the end it should say similar to:
[...] usb 3-10.3: new full-speed USB device number 87 using xhci_hcd
[...] usb 3-10.3: New USB device found, idVendor=1a86, idProduct=7523
[...] usb 3-10.3: New USB device strings: Mfr=0, Product=2, SerialNumber=0
[...] usb 3-10.3: Product: USB2.0-Serial
[...] ch341 3-10.3:1.0: ch341-uart converter detected
[...] usb 3-10.3: ch341-uart converter now attached to ttyUSB2
Verify with command "lsusb":
Bus XXX Device YYY: ID 1a86:7523 QinHeng Electronics HL-340 USB-Serial adapter
Check available devices with "ls -al /dev/ttyU*" to get something like:
crw-rw---- 1 root dialout 188, 0 Dez 16 10:13 /dev/ttyUSB0
crw-rw-rw- 1 root dialout 188, 2 Dez 16 10:05 /dev/ttyUSB2
If you get more than 1 line, you need to determine which one it is. The dmesg command tells you the device is /dev/ttyUSB2.
Alternatively, install GeigerLog https://sourceforge.net/projects/geigerlog/ and run its Help --> Autodiscover connected USB port.