Welcome to the Two Wheeled Texans community! Feel free to hang out and lurk as long as you like. However, we would like to encourage you to register so that you can join the community and use the numerous features on the site. After registering, don't forget to post up an introduction!
They are tires designed for real dirt bike rims. 1.6 inch wide rims for the front (up to 1.85 would be OK) and 2.15 inch on the rear, would be OK up to a 2.5 inch rim on the rear. I know nothing very specific about the rim width of a 800GS, but I do know they do not come with narrow dirt bike rims. If you have aftermarket narrow wheels these tires will be OK for you ride. They will not be necessary though. In dry conditions, any dual sport tire designed for big Adventure bikes will be fine for what you will be riding. It is a pretty mild ride as far as technicality is concerned, assuming basic competency for riding skill. If it really wet, there are sections of the NMBDR that no tire will save you.
These tires were never designed for a 500 pound Adventure bike. As far as wear, the front will make 1200 miles but will be done. Hard to say on the rear because it depends how you ride the bike. It may make 1200 miles. Either way, no way I would mount it up on a stock 4 inch wide rim, or whatever the width is on your bike.
I will say no on the rear if you ride spirited at all. I ran them on my KTM 525 once. The front lasted fine the back not so much. I don’t keep mileage records really, but I can assure you it didn’t make 1000 miles.
Thanks guys. The front and rear wheels are from Woodies, but based on what you’re saying I doubt the rear will make it. Maybe I’ll try to burn use them up on the next Big Bend trip or give them to someone whose bike is better suited.