I have a 6x4 Trailer with the above mentioned mod to the headboard and a custom alloy front wheel brace. I can get my 990 Adv into it (although I can't quite close the tail. (Which is quick release anyway)
It will however comfortably take an F800ST with the tail closed.
I have a ramp for it too.
You're more than welcome to borrow it if you like?
Same size as my trailor. I have taken an old BSA super rocket on it several times before but this is much smaller and lighter than GS ,so from my experience I would definately not try it.