I looked on my 1200 GS manual and only one first gear ratio is listed (also checked the 1200RT) I would have thought all you need to order a part is the part number and it should arrive through the system. I realise getting the part number is the hardest part, have you tried the German GS forum.
How sure are you that these gears exist? Pre 1200 (11**) I think the way to lower the gearing was by fitting different final drive, I not saying this is the way to go as you lower the overall gearing.
I might just suggest you take a moment to consider what you’re trying to achieve. I have ridden the 1200 GS in muddy and sandy conditions and a lower first gear would be handy, in such conditions I advocate the slow and steady approach on such a heavy bike. I have witnessed ( and tried) the fast and furious approach and it often goes wrong, and while amusing to watch the consequences for the bike and rider and often disastrous. However, how much will you need the lower first, it's going to cost estimated £500 plus.
I had a KTM 640 adventurer and that has a low first gear and it did not make it a joy to ride in tight fast corners on the road, first was too low and second too high. But it did make a good bike in the worse off road conditions. I also carried a larger front sprocket for faster road work. 15 minutes to change over the front sprocket.
If you are going to be riding a lot in the conditions that need a low first gear maybe you need to consider if the 1200 is the right bike.

Only trying to play devils advocate here

I have listed the ratios for the 1200 below incase you have'nt seen them.

Steve

Primary transmission ratio
1.824 - 31 / 17 teeth

Gear ratio, 1st gear
2.277 - 41 / 18 teeth

Gear ratio, 2nd gear
1.583 - 38 / 24 teeth

Gear ratio, 3rd gear
1.259 - 34 / 27 teeth

Gear ratio, 4th gear
1.033 - 31 / 30 teeth

Gear ratio, 5th gear
0.903 - 28 / 31 teeth

Gear ratio, 6th gear
0.805 - 29 / 36 teeth