[Question] Spacing in amsmath environments

[antshar]

How is the spacing between rows being calculated in gather, align etc. environments calculated?

I just want to figure out the dimension of row sep

\documentclass{article}
\usepackage{amsmath}
\setlength{\fboxsep}{0pt}
\setlength{\fboxrule}{.1pt}
\begin{document}

\begin{gather}
\fbox{$a=b$} \\
\fbox{$\dfrac22$}
\end{gather}

\end{document}

I tried to look up the docs, but it appears to be hidden somewhere so I couldn't find it myself.