We propose to study the impact of gate width selection on $\gamma$-ray spectrum parameters such as peak-to-background ratio and statistics. We will approach this study on both theoretical and experimental point of view. Starting from the results of this preliminary study a new method of background subtraction has been elaborated. It is designed to remove efficiently contamination and continuum component in $\gamma$-ray spectrum. These works will be illustrated more specifically in the case of superdeformation for the $^{151}$Tb nucleus.