The COM port driver can be downloaded separately
downloads/CPS/Com%20port%20driver%20for ... tallation/
Unzip the file and try running the install_OpenGD77_comm_driver.bat
If that does not work, you could try going into the Windows Device manager, selecting the unnamed USB device and select the OpenGD77-serial-driver-files, however Windows generally does not allow users to manually install drivers this way.
Unfortunately from Windows XP onwards, Microsoft started locking out non-commercial drivers, but some clever people developed a workaround called LibWDI
The drivers used by OpenGD77 are actually the ArduinoSTM32 comm port drivers, renamed to OpenGD77. I'm allowed to do this as I'm also owner of the legacy ArduinoSTM32 project.