Bottom "overflow" over matrix delimiters

[mazmap]

With the original gauss.sty code,

\begin{align*}
  \begin{gmatrix}[p]
    3 & \displaystyle\sum_{i=1}^n & 5 \\
    1 & 2 & \displaystyle\frac{1}{2}
  \end{gmatrix}
\end{align*}

renders

whilst just using pmatrix from AMS LaTeX, i.e.

\begin{align*}
  \begin{pmatrix}
    3 & \displaystyle\sum_{i=1}^n & 5 \\
    1 & 2 & \displaystyle\frac{1}{2}
  \end{pmatrix}
\end{align*}

yields

Both do not appear to be optimally rendered but the gauss.sty example is significantly worse than the AMS LaTeX one. This overflow should be fixed.