\documentclass[a4paper,12pt,openany,oneside]{MsThesis}
\input{commands}
\usepackage{mathtools}
\begin{document}
\begin{align*}
G(y)= \left\{\begin{array}{l}
0,\qquad  y<0,\\
1-e^{-y}, \quad y \geq 0,
\end{array}\right.  \\
\end{align*}
\end{document}