I've got the BMW tank on my 1100GS. You can squeeze in 31L from empty.
Currently in Nepal and it's given me a range of up to 600km max.
You should bear in mind the extra weight and bulk of larger fuel tanks(when full), specially when trying to manoevre a heavily laden GS.
On my trip a standard fuel tank would probably have sufficed but the bigger one was handy for allowing us to be more flexible with when to stop. I could only envisage needing a bigger tank for really remote areas. The TT, as suggested, seems to fall far short of its claimed capacity anyway.
I got my tank off eBay for 365 quid but have seen them cheaper too.