I think the problem here is that both USB and serial interfaces can be master or slave (controller or controlled), eg. a computer USB port would be a master and a USB card reader would be a slave, on the uniden the serial GPS port is the master and the 60sx is the slave so the uniden controls the transfer of data, however if you connect the uniden serial port to a serial - USB converter and then connect the converter USB to the 64 you are connecting a slave (the converter) to a slave (the 64)
When the uniden was connected to the 60csx there was a serial port (master) talking to a serial port (slave) with the uniden starting the conversation, but if you have the uniden serial port (master) connected to a serial - USB converter (slave) and then to the 64 (slave), the uniden will try to start a conversation but it won't control the serial - USB converter as it doesn't expect it to be there, it would normally be controlled by a computer (master) on the USB side, but in this case the USB side is connected to the 64 which is a slave