Once upon a time, I installed an Ohlins shock to my GS.  The Ohlins part number for the GS shock is  BM 344, Type 46ERS.  It was a great shock... much improved over the stock shock... just no comparison really (original pictures from that installation here).

With all of the significant upgrades to my bike and the front suspension, I decided to complete the project (in terms of suspension anyway) by adding an Ohlins for the Dakar,  part number BM048, type 46HRCS, which will replace the previous Ohlins.

Here is a comparison picture between the stock GS shock and the Dakar shock:

Picture borrowed from F650.com

The most visible difference (aside from color) is the reservoir.  

The F650 Dakar has a bracket welded to the frame for mounting the reservoir (the Dakar has a shock with an external reservoir from the factory); whereas, the GS has no bracket (because the GS doesn't come with that kind of shock).  

On F650.com, I found the following "solution":

Picture borrowed from F650.com

I guess you don't have to be a genius to see the design flaw with this option.  I mean, come on... 2 skinny zip-ties to hold a cylindrical-shaped reservoir to a rectangular frame... .  That would last about 10 minutes with me.

Since I was faced with the same challenge, I came up with a solution...

Basically it is a small custom bracket with two saddles for holding the cylindrical shape of the reservoir, 2 - 5mm bolts, 2 - 5mm nut-zerts, 2 - 5mm fender washers, 2 - 5mm flat washers, 2 - "zebra" straps, 2 pieces of heat shrink, and two small pieces of rubber to go between the saddle and the reservoir.  

Here's a sketch of what you see above...

The "zebra" straps holding the reservoir on are covered with the black heat shrink so they look nicer and protect the finish on the reservoir from the metal-on-metal contact.

You will need an angle drill and a very short drill bit to drill out the holes for the nut zerts.  

So, if you have a GS and want to install a shock with an external reservoir, perhaps this is an option for you.