Too much space at the top of a beamer block when using multilined math environment

You can set the length abovedisplayskip to 0pt (or any other desired value):

\documentclass{beamer}
\usepackage{amsmath}
\begin{document}

\begin{frame}
\begin{block}{Separation of variables}
\begin{gather*}
 u'''' - (yu')' = \lambda u,\\
u''(0)=u'''(0)=0 \quad \text{(Free),} \qquad u(1)=u''(1)=0 \quad\text{(Clamped)}
\end{gather*}
\end{block}
\begin{block}{Separation of variables}
\setlength\abovedisplayskip{0pt}
\begin{gather*}
 u'''' - (yu')' = \lambda u,\\
u''(0)=u'''(0)=0 \quad \text{(Free),} \qquad u(1)=u''(1)=0 \quad\text{(Clamped)}
\end{gather*}
\end{block}
\end{frame}
\end{document}

Of course, you can use something like

\addtobeamertemplate{block begin}{\setlength\abovedisplayskip{0pt}}

right after \begin{document} to apply the change to all the blocks.

A crude, but simple way is to insert \vspace{-.5cm} (or whatever negative distance suits you) just before starting the gather environment. Most likely someone knows a better way than this quick-and-dirty workaroumd... :)