The current one obtainable from the dealers is AD, anything above a L will be fine
Use VCDS before removing the old gateway and note down everything in device 19, write down the code number and the descriptions that are enabled or screen dump them, check the car type in long coding help on the last byte
Install the new gateway, and select the same options in the new gateway (the code numbers and descriptions may not be in the same order). Then in long coding helper, set the car type in the last byte
clear all the errors
go for a drive
don't worry if you lose your auto dimming rear view mirror if fitted and rear wiper, after about 20 mins of driving the car will rediscover them all by itself