Center single line(s) in align

To have more or less the spacing of the original code, I'd use the alignedat environment, to have full control on the spacing:

\documentclass{article}
\usepackage{mathtools}

\begin{document}

\begin{gather*}
0 = 0\\
\begin{alignedat}{2}
a &= b &\hspace{10em} c &= d\\
a &= b & c &= d\\
a &= b & c &= d\\
a &= b & c &= d
\end{alignedat}
\end{gather*}

\end{document} 

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{gather*}
0 = 0\\
\begin{aligned}
a &= b & c &= d\\
a &= b & c &= d\\
a &= b & c &= d\\
a &= b & c &= d
\end{aligned}
\end{gather*}

\end{document}