The "wet" lead-acid battery on my '02 R1150GS Adventure was getting a little long in the tooth (3+ years), so I decided that while doing some other work (ahem) on the bike, I would just go ahead and replace the battery. The tank was already off, so it was a relatively easy process.
A trip to the Batteries Plus website revealed that they carried the Odyssey PC680MJ-A which is recommended for the R1150GS Adventure. The "MJ" portion of the part number stands for "Metal Jack" and that's necessary to protect the plastic case of the battery from the heat and vibration that it's subjected to on the Adventure - or so they say.
I stopped by the local Batteries Plus store to see if they had the battery in stock. No such luck, but I wasn't disappointed. In fact, I was pleasantly surprised to find that my "company" discount provided nearly a $40 discount on this rather pricy battery. I promptly ordered one up and was able to pick it us within a week for about $120 before tax. Sweet.
The metal jacket is essentially a metal box that the battery slips into. The plastic battery case is actually black in color and can be seen above the jacket near the top of the battery.
The mounting posts on the original battery are substantially different, so you need to make sure that you obtain a set of L-bracket hardware to allow the stock R1150GS Adventure battery cables to connect without major modification. Follow the torque specification provided when fastening the L-brackets to the battery posts.
Note the orientation of the L-brackets in the picture below. From what I can tell, this is the ONLY way to make things fit without modification to the battery cables on the R1150 GS. I had to remove one of the bends on ground-side terminal with a Dremel Tool, but it was just a minor modification. Why you're at it, take the time to clean all your connections up. I had some serious corrosion on the positive side that took some time to clean away on the accessory ring connectors.
You'll find the metal jacketed battery a tight fit in the stock battery box on the GS. With the old battery out, take some time to clean up in and around the box and take your time when sliding/pushing the new battery into place. You won't be able to prevent scratching up that pretty orange box, so don't worry about it.
Once the battery has been installed, place it on your trickle-charger right away and get it up to proper voltage. I found quite unscientifically that the battery I received required some charging to 100% before first use. No worries and it's quite happy now. Many folks recommend charging they (and other) cycle batteries with a small maintenance charger whenever the bike is parked. I'm not that religious about it, but do rotate the charger around the garage from time to time.