Depending on what model G8 you have (If you have the colour LCD screen fitted or not), there may be a couple of things to look out for.
Firstly the Radio frequency and and the knob positions are no biggie, as there is a hidden service menu where you can set these, without resorting to Tech2, EEPROM hacking
Note the Frequency and the RHD options
If you have the non colour LCD display model G8, then you will have the non climate control HVAC system.
(So you will need to source a Dual Zone Climate Control module, and hack it to think you have the required sensors for DZ operation).
Finally you will need the head unit programmed. (You will need to VIN match the headunit to your cars VIN number)
The easiest method would be to source an EEPROM programmer, download the current EEPROM, modify the VIN to your own vehicles, then upload back.
Info on hacking the HVAC Module.
www.carmodder.com • Converting to Automatic Climate Control for the Holden VE Commodore
And Info on programming the Headunit EEPROM.
www.carmodder.com • Programming the Colour LCD HVAC / Radio for the Holden VE Commodore
Any reason why you want a Calais headunit ?
(The only real difference between the Oz Calais headunit and the GXP G8 headunit is the NAV / Power button, and LED illumination)
Hope this helps ?