The general rule is that if your car is earlier than a '07' registration plate then you will most likely need a new CAN Gateway. Mk5 Golfs were fitted with gateways with the part number
1K0 907 530 followed by a letter revision, i.e.
1K0 907 530 L. If your car has a revision 'L' or earlier then it will suffer battery drain, caused by the radio not shutting off its supply from the battery.
To find out your CAN Gateway revision/age you can scan the car with VCDS (VAG-Com) or physically locate the CAN Gateway (under the driver's side dashboard) and loo at the part number on the sticker.
Any gateway newer than revision 'L' will solve battery drain. Once VW got to 1K0 907 530 Revision 'Z' they started back at 'AA, AB, AC...'. Any of these are newer so will suffice.
'1K' refers to the Mk5 Golf however there are other part numbers of CAN Gateway that will fit the Golf as well. Eddie-NL on here sells brand new 7N0 907 530 Gateways (7N refers to the VWE Sharan). As far as I know these are all new enough to prevent battery drain, no matter what revision they are.
People have many questions about the CAN Gateway as there are many of them (revisions/variants) and several locations on the Golf where VW put them - up behind the clutch pedal, down underneath the stereo or above the accelerator pedal.
Therefore there are several ways to remove them. You need to go looking at your own car before deciding which one to do.
BEFORE you remove the Gateway you need to have it scanned using VCDS (VAG-Com) and take note of all the options ticked (series of tick boxes). Then you can buy a 'blank' un-coded Gateway and code it yourself (re-tick all the boxes, plus anything you want to add i.e. Aux In, etc.).
Or if you have the list of installed items (ticked boxes) you can send this to Eddie-NL along with the appropriate cash and he can usually deliver you one of the latest '7N0' Gateways.
Hope this is helpful, seems there are about a dozen threads all asking the same questions about Gateways, etc.
Coincidentally I have the following for sale:
RCD510:
http://www.mk5golfgti.co.uk/forum/index.php/topic,79957.msg830488.html#msg830488RCD510 DAB:
http://www.mk5golfgti.co.uk/forum/index.php/topic,79992.msg830926.html#msg830926MDI Armrest Dock:
http://www.mk5golfgti.co.uk/forum/index.php/topic,79958.0.html