Hi,
I think you'll need the gadget.... As the RCD300 radio on the Golf V appears to be completely controlled by the CAN bus.
The only connectors are speakers, power and ground, plus an alarm lead, plus the CANbus twisted pair wiring. The CANbus is used for "lights on", "radio on" and GALA - there is no switched ignition wire, nor is there a lights on cable.
The connectors on the back appear similar to the VW Delta unit - not the Gamma.
Colin