Adjusting the left margin of glossaries

Question

Alright, let's try this working example:

\documentclass{article}

\usepackage[acronyms,nonumberlist,nogroupskip,nopostdot,toc]{glossaries}
\usepackage{glossary-mcols}

\renewcommand{\glsmcols}{2}
\renewcommand{\glossarypreamble}{\glsfindwidesttoplevelname[\currentglossary]}
\setglossary{mcolalttree}
\renewcommand*{\glossaryname}{Nomenclature}
\renewcommand*{\acronymname}{Abbreviations}

\makeglossaries

\begin{document}

\printglossaries

\newglossaryentry{pt}{name=$\sfrac{P_{T2}}{P_{T0}}$, description={Ratio of total pressure at AIP to freestream total pressure}}
\newglossaryentry{y+}{name=$y^{+}$, description={Dimensionless distance from wall boundaries}}
\newglossaryentry{d}{name=$D$, description={AIP diameter}}
\newglossaryentry{m2}{name=$M_2$, description={Mach number at AIP}}%
\newglossaryentry{pt2avg}{name=$P_{T2_{avg}}$, description={Average of 40 AIP Kulite\textsuperscript{\textregistered} total pressures}}%
\newglossaryentry{p2avg}{name=$P_{2_{avg}}$, description={Average of 8 AIP static tap pressures}}
\newglossaryentry{ds}{name=$\Delta s$, description={Distance measured between neighboring grid points}}
\newglossaryentry{dpcp}{name=DPCP, description={Circumferential distortion intensity}}
\newglossaryentry{dprp}{name=DPRP, description={Radial distortion intensity}}
\newglossaryentry{pav}{name=PAV, description={Average ring pressure}}
\newglossaryentry{pavlow}{name=PAVLOW, description={Average low pressure}}  \gls{pavlow}

\newacronym{aip}{AIP}{Aerodynamic Interface Plane}\gls{aip}
\newacronym{amr}{AMR}{Adaptive Mesh Refinement}
\newacronym{gcr}{GCR}{Generalized Conjugate Residual}

\glsaddall

\end{document}

Now I would like to adjust the margins on either side of the glossary so that both left and right margins are larger by about 3em.

Answer 1

You can put the entire glossary inside a list and adjust the list margins like this:

\documentclass{article}

\usepackage[acronyms,nonumberlist,nogroupskip,nopostdot,toc]{glossaries}
\usepackage{glossary-mcols}

\renewcommand{\glsmcols}{2}
\setglossarystyle{mcolalttree}
\renewcommand{\glossarypreamble}{%
 \glsfindwidesttoplevelname[\currentglossary]
 \begin{list}{}%
 {\setlength{\leftmargin}{3em}\rightmargin\leftmargin}% margins
 \item\relax}
\renewcommand{\glossarypostamble}{\end{list}}
\renewcommand*{\glossaryname}{Nomenclature}
\renewcommand*{\acronymname}{Abbreviations}


\makeglossaries

\begin{document}

\printglossaries

\newglossaryentry{pt}{name=$\frac{P_{T2}}{P_{T0}}$, description={Ratio of total pressure at AIP to freestream total pressure}}
\newglossaryentry{y+}{name=$y^{+}$, description={Dimensionless distance from wall boundaries}}
\newglossaryentry{d}{name=$D$, description={AIP diameter}}
\newglossaryentry{m2}{name=$M_2$, description={Mach number at AIP}}%
\newglossaryentry{pt2avg}{name=$P_{T2_{avg}}$, description={Average of 40 AIP Kulite\textsuperscript{\textregistered} total pressures}}%
\newglossaryentry{p2avg}{name=$P_{2_{avg}}$, description={Average of 8 AIP static tap pressures}}
\newglossaryentry{ds}{name=$\Delta s$, description={Distance measured between neighboring grid points}}
\newglossaryentry{dpcp}{name=DPCP, description={Circumferential distortion intensity}}
\newglossaryentry{dprp}{name=DPRP, description={Radial distortion intensity}}
\newglossaryentry{pav}{name=PAV, description={Average ring pressure}}
\newglossaryentry{pavlow}{name=PAVLOW, description={Average low pressure}}  \gls{pavlow}

\newacronym{aip}{AIP}{Aerodynamic Interface Plane}\gls{aip}
\newacronym{amr}{AMR}{Adaptive Mesh Refinement}
\newacronym{gcr}{GCR}{Generalized Conjugate Residual}

\glsaddall

\end{document}

The gap between columns can be adjusted by changing \columnsep.

Unrelated, but I recommend you move your definitions to the preamble.