Remove the option numberedsection
if the glossary title should be a numbered section but using \section*
.
\documentclass{article}
\usepackage[portuguese]{babel}
\usepackage[utf8]{inputenc}
\usepackage[nopostdot,nonumberlist]{glossaries}
\makenoidxglossaries
\newglossaryentry{foo}{%
name={Foo},
description={Successful foo}
}
\begin{document}
\section{First}
\subsection{Glossário} % Not needed actually
\glsaddall
\setglossarystyle{altlistgroup}
\printnoidxglossary
\end{document}

Update
Using section=subsection
and numberedsection
is probably what the O.P. requests:
\documentclass{article}
\usepackage[portuguese]{babel}
\usepackage[utf8]{inputenc}
\usepackage[nopostdot,nonumberlist,section=subsection,numberedsection]{glossaries}
\makenoidxglossaries
\newglossaryentry{foo}{%
name={Foo},
description={Successful foo}
}
\begin{document}
\section{First}
\glsaddall
\setglossarystyle{altlistgroup}
\printnoidxglossary
\end{document}

numberedsection
as an option