"Psych!" (6)
(Other definitions for gotcha that I've seen before include "I have found you" , "You're caught - I understand you" , "You've been caught" , "cry of apprehension" , "claim to have made conquest?" .)
(Other definitions for not that I've seen before include "Ton (anag.)" , "logically, operation needed" , "Negation of what follows" , "Word of denial" , "It negates a word" .)