There are a number of ways to improve on this. Most important error is to not multiply trapezoid heights by step length. The value below is pretty good for an approximation.
step*(Total[Map[gausiana, N[PuntosEnsayo[[2 ;; -2]]]]] +
1/2 (gausiana[PuntosEnsayo[[1]]] + gausiana[PuntosEnsayo[[-1]]]))
(* Out[192]= 0.0000718038661611 *)