My motorcycle was leaking gasoline until I was unscrewing and screwing the screw where it was leaking  now it stopped leaking? ?

You must not want a valid answer, since you failed to say what motorcycle you have, and made the description and location of the screw as mysterious as a Sherlock Holmes novel. Get a repair manual.

The screw was relevant to how something you didn't describe well is put together. Tightening the screw took care of that. But keep an eye out for more leaks at that spot, and try to understand exactly what is leaking.

You were lucky this time. If you don't know why you're turning a screw you're usually well on your way to making things worse. Get to know the machine you're "playing" with and your fixes will stick.

I cant think of anything  "behind" an engine where gas Should  go. Can you post a picture?

Fact it stopped Now is good. It was jarred loose  when she dropped. 

You are probably talking about the carb drain screw.  It was loose and you tightened it.  If it leaks again, remove the screw and put a new o-ring on it.