Typical GX6605S PCB UART pinout:
The selected COM port is wrong or in use by another software.
[PC with GXDownloader] ---> (USB) ---> [TTL Adapter] ---> (Serial RX/TX) ---> [Receiver Mainboard]
The utility runs natively on Windows XP, 7, 8, 10, and 11. Software Files GXDownloader Boot V1.032: The core executable application.