how i execute function after i release button ?

Please don't post duplicate questions. If you don't like answers you received, respond to the answer and explain why it does not work for you. Given your your question, @jamiltron answer seems sound. In particular, GUI.Button() calls execute on the mouse up, not the mouse down. If you mean something else...like have Shoot execute at some specific time after the button is released you can use Invoke:

 if ( GUI.Button=(Rect (100,200,50,100), "Shoot") ) { 
     Invoke("shoot", 0.5);
 }