I've only test ridden the F800GS a few days, never traveled too far but did get to ride it off road a bit. I really liked the F800GS but in more challenging off road it won't come close to my basic DR650 ... which is 45 kg lighter weight.

But the GS is MOST definitely the bomb on road. I loved it! Good handler, good brakes, smooth and fast enough. Probably just about perfect for about 80% of what you might find going RTW. It's that 20% bit that's worrying.

If you go looking for more challenging tracks with deep sand, mud, rocky, steep up or down, then the 800 could be hard to manage, but lots depends on YOU as a rider ... and on how much of a load your packing on the bike.

I did feel the weight when riding the GS in the easy off road I did, but I'm a little guy and bit out of shape. But I was a "B" level AMA Enduro rider ... I'm good off road but out of shape and weak now.

A rider with skills and good strength should do fine off road. The Skills part being most important!

In the history of the F800GS have read about some strange engine failures on this bike over the years. Hope all that has been solved??
Is there any remaining warranty with the bike you're considering? IMO, the warranty is the very BEST thing about BMW! I would never buy one without plenty of warranty remaining.

All the BMW's I have tested (back to late 90's), always preferred standard GS model over ADV model.
Problem is ADV model adds so much weight.

But if you need the crash guards, boxes and bigger tank, then probably worth it for travel. For me, the bike was already at my weight/strength limit so adding the weight of the ADV model was a No Go.

But I'm not riding the bike RTW ... so there is that!

I say GO FOR IT!