I would suggest the easiest way (what I do) is have a motorbike-specific one that plugs into the BMW accessory socket. It seems easiest for me. It's canbus aware (the socket on the bike) so I would treat with care (i.e. only plug something designed for the job).
Fit an auxiliary socket direct to the battery (with a suitable fuse).
Then you can use any charger.
You can also use the socket for high current devices like tyre pumps.
There are many threads on this topic, for every version of the GS ever made. Same reply.
Do you have a smart charger already? Is it AGM battery compatible? If yes, fit a fly lead directly to the battery and use your existing charger. This will eliminate any anxiety re. the canbus plus you can use the fly lead to power heavy duty accessories like a tyre pump or heated clothing. The canbus controlled socket has a 5 Amp limit.
If you need a new charger then the Wunderlich branded Optimate 4 mentioned above is good but out of stock