I enquired a couple of years ago (2006MY) about this when I first placed an order for a GT. I had a Webasto parking heater(*) retro fitted to my previous Land Rover Freelander and loved it.
As many turbo diesel cars have the heater part of one of these fitted it is an easy task to add the pump and timer to turn it into a parking heater. Inquiries by the Webasto agent, Vehvac - Edenbridge, Kent, got only a "Don't know" reply from AR UK!
My 2007MY GT JTdm doesn't seem to warm up that quickly and I would say it isn't fitted.
Although it's an old version (2004) the Alfa parts catalogue ePer doesn't show a pre heater on any model except some 156s.
If anyone knows where the pre heater is located I'll have a look to see if its a Webasto or possibly Ebarspacher unit. If so I think its about £200 to fit the pump and timer to convert to full Parking Heater Spec'........
(* - you can set a parking heater timer to come on a short time before you leave home/work. The fuel powered burner fires up, heats the engine coolant to temperature and then turns on the blowers to heat the interior/ demist the screen. You step into a warmed up engine and heated/ demisted interior! (You can also spec' the Webasto to start by radio remote control or by phoning a pager number (the unit can have a mobile phone/ pager module built in!)