How to indent subsection in AMS TOC

The class gsm-l is based on amsbook and subsection entries in the ToC are typeset according to \l@subsection from amsbook with definition:

\def\l@subsection{\@tocline{2}{0pt}{1pc}{5pc}{}}

The third mandatory argument for \@tocline controls the indent for the entries; change 1pt to 2.5pc (or to the value that suits your needs):

\documentclass{gsm-l}

\setcounter{tocdepth}{3}
\makeatletter
\def\l@subsection{\@tocline{2}{0pt}{2.5pc}{5pc}{}}
\makeatother

\begin{document}

\tableofcontents
\chapter{Test chapter}
\section{Test section}
\subsection{Test subsection}
\subsection{Test subsection}
\section{Test section}
\subsection{Test subsection}
\subsection{Test subsection}

\end{document}

Perhaps you might also want to change the fourth argument (controlling the hanging indentation for entries spanning more than one line), so perhaps

\def\l@subsection{\@tocline{2}{0pt}{2.5pc}{4pc}{}}

could also be a sensible redefinition.

FWIW, I use subsubsections in my writing and the currently accepted answer doesn't behave as one would like when there are additional indentations that need to be made.

If you're in the same boat as me, use this code to make subsubsections be indented beyond subsections in the ToC!

\setcounter{tocdepth}{3}
\makeatletter
\def\l@subsection{\@tocline{2}{0pt}{2.5pc}{5pc}{}}
\def\l@subsubsection{\@tocline{2}{0pt}{5pc}{7.5pc}{}}
\makeatother