My friend rode shes GS with full tank for only 20miles and the tank was totally empty absolutely no fuel at all  :x ...
What a hell can suck up that much of fuel??
That makes only 4mpg!!!

It can't.  I think a GS would literally drown in fuel if it were pumping that much into it.

Refill it and look for leaks.

Maybe open fuel valve (petcock or smth) could cause that leak, like over floated carbs or something?
Or heat? Can heat about +25 per celsius cause some kind of vapourizing?

Not unless you've taken a can opener to the top of the tank.  

It might have a stuck float valve but I think it would tend to flood the cylinder instead of getting dumped overboard like it should.

What and your friend has no nose...The smell of gas in all those cases will be ridiculous. I think you lost fuel flow, there is still fuel in the tank. If the tank is really empty (verified by looking inside it) then its leaking abut 4 gallons in 20 mins is a river...Just smell it and it tells you.
Cool.
Srinath.

Yep, check to see if your engine case is full of gas. If it is, how does it handle? Gas weighs about 6lbs per gallon, so you would have a 24lbs shift in weight and center of gravity would be lower, too. And, to know that the cases can hold four gallons!

Sorry, I don't mean to be a smart ass, I hope I didn't offend.

at that rate you would be inhalling a lot of fumes, maybe huffing that much gas you "thought" it was 20 miles!

sorry, hd to throw in my .02 smart ass comment too!

Are you filling it all the way up the filler neck? Or, just stuffing the entire filler nozzle into the tank like you would do to a car?

Something is up.  If that much gas went into the engine or cases, the bike would break down. It would be flooded or otherwise mucked up.  

If that much gas was leaking at the petcock, there would be raw fuel literally streaming onto a very hot engine.  I'd imagine something like that would make itself known one way or another.  

I'm guessing that the tank is either not really empty now, or it was never full in the first place.