The command\geometryget takes a single mandatory argument, which should be the name of one of geometry package's optional parameters, e.g. left, top, textheight, etc. (the version below supports only the parameters lmargin, rmargin, tmargin, and bmargin, and their various aliases, but it can be readily extended to include all of geometry's optional parameters).
It expands to the value of the corresponding parameter, in units of pts.
\documentclass{article}
\usepackage{pgfkeys}
\pgfkeyssetvalue{/geometry/lmargin}{lmargin}
\pgfkeyssetvalue{/geometry/left}{lmargin}
\pgfkeyssetvalue{/geometry/inner}{lmargin}
\pgfkeyssetvalue{/geometry/innermargin}{lmargin}
\pgfkeyssetvalue{/geometry/rmargin}{rmargin}
\pgfkeyssetvalue{/geometry/right}{rmargin}
\pgfkeyssetvalue{/geometry/outer}{rmargin}
\pgfkeyssetvalue{/geometry/outermargin}{rmargin}
\pgfkeyssetvalue{/geometry/tmargin}{tmargin}
\pgfkeyssetvalue{/geometry/top}{tmargin}
\pgfkeyssetvalue{/geometry/bmargin}{bmargin}
\pgfkeyssetvalue{/geometry/bottom}{bmargin}
\newcommand{\geometryget}[1]
{\csname Gm@\pgfkeysvalueof{/geometry/#1}\endcsname}
\usepackage{geometry}
\begin{document}
\begin{tabular}{|ll|ll|ll|ll|}
lmargin&\geometryget{lmargin}&
rmargin&\geometryget{rmargin}&
tmargin&\geometryget{tmargin}&
bmargin&\geometryget{bmargin}\
left&\geometryget{left}&
outer&\geometryget{outer}&
top&\geometryget{top}&
bottom&\geometryget{bottom}
\end{tabular}
\end{document}

A more substantial example of usage.
The following example uses \geometryget to set the width of the margin notes to .8 of the width of the right margin and to center them in the right margin.
In the code above insert the following to the preamble after the loading of the geometry package.
\geometry{hscale=.66,hmarginratio=1:4}
\usepackage{pgfmath}
\pgfmathsetlength{\marginparsep}{.1\geometryget{right}}
\pgfmathsetlength{\marginparwidth}{.8\geometryget{right}}
\usepackage{lipsum}
Replace the document's body with the following:
\lipsum[1-1]
\marginpar{\lipsum[2-2][1-1]}
\lipsum[3-7]
