I found a trick somewhere to upload to an Arduino Uno using only UART TX/RX from a PL2303 USB interface: hold the reset button of an Arduino Une for 2-3 seconds starting just before you hit the upload button. It works with an Arduino Uno clone that I have with a defective USB port, and I hoped that a similar trick would be available for the Gertboard. But I already sent my Gertboard back as it was defective, so no way for me to try out. Now that I think of it, I don't think it had a reset button.