The front panel of the box should change from "BOOT" to "USB" or "Lxxx" (indicating a load percentage). Do not turn off the power during this time.
You will rarely find a single generic "Gxrom.bin" download that works for every device. Instead, Gxrom.bin is usually a of your receiver's specific firmware. Gxrom.bin Download
Copy it directly to the root of the USB drive (do not put it inside any folders). Turn off the receiver's power. Insert the USB drive. The front panel of the box should change