If you want a big ADV bike but fire trails are the toughest you'll get onto, then all of them, save maybe the Ducati Multistrada and the Kawi Versys 1000 (both of which are distinctly STREET bikes!)... You're not hurting for choices!
- BMW R1200GS/R1200GS Adventure
- Ducati Multistrada 1200 / Multistrada 1200 Enduro
- Suzuki V-Strom 1000 / V-Strom 1000 XT
- Aprilia Caponord
- KTM 1090 Adventure
- KTM 1290 Adventure
- Honda VFR1200X
- Yamaha Super Tenere
- Triumph Tiger Explorer
- Honda Africa Twin
- Moto Guzzi Stelvio NTX
- Kawasaki Versys 1000
Here's the fucking great part... no matter what you get, you're bound to be pretty happy with it, barring some sort of catastrophic failure. ALL of them are capable bikes in any circumstances you'll deal with on the road. Most of them will do fine on dirt and gravel. None of them will be spectacular on really tight or soft stuff without an awful lot of practice, and it takes a lot of work to do relative to a real off-road bike. And many of them (Tenere and V-Strom) will need some sort of under-engine protection for long trips over gravel to prevent the engine and oil filters from being beaten to hell and back.
I'm on a 2013 Super Tenere. It's a stellar motorcycle. I've owned 14, this is the best by a huge margin. I have no qualms about it being a pretty poor dirtbike... it's just a gigantic, porky, ultra-durable UJM to me. Upright seating, excellent front end feel, great brakes, very good suspension (even on my basic model), great mid-range kick, and I got it brand new for a song. The seat is, frankly, amazing for a factory item. And... the newest ones are even better in pretty much every way!
As for the R1200GS/A, if you won't be doing heavy off-road riding, you may be better off with the base model, with its road-oriented tires and lower seat height... and lower price, of course. As for the other models, it's really a matter of prioritization. For a sportbike guy wanting something upright, the MTS is the best, by far. For someone getting dirty, a Africa Twin, GS-A, or one of the KTMs might be the way to go.
Each model has its own collection of high points and compromises. But none of the compromises (aside from weight) are going to stop you from doing most things you want to do with them... I don't think there's a bad bike in the lot of them. Of any model I remembered to list above, I'd gladly own/ride any of them.