For an unknown reason the documentation in CTAN is out-of-date. I have uploaded the most recent version.
There is no problem using Adobe Garamond Pro with pdfLaTeX.
\documentclass{article}
\usepackage[Adobe Garamond]{mathdesign}
\begin{document}
This is a test: $x+y=2$
\end{document}
But for the moment, mathdesign doesn't offer Unicode fonts.
I just figured out, that you can solve the problem by using Garamondx as your font instead of Garamond. You can download and install it from CTAN. The MWE looks like this:
\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[ngerman]{babel}
\renewcommand{\rmdefault}{ugm} % URW Garamond
\renewcommand{\sfdefault}{uop} % URW Classico (Optima)
\usepackage{luximono}
\usepackage[garamond]{mathdesign}
\usepackage{garamondx}
\begin{document}
\noindent
\normalfont RM-Family: Standard, emph, textbf, textit, textsl, texttt, textsc\\
\rmfamily Text, \emph{Text}, \textbf{Text}, \textit{Text}, \textsl{Text}, \texttt{Text}, "`Not available"'\\
\noindent
\normalfont SF-Family: Standard, emph, textbf, textit, textsl, texttt, textsc\\
\sffamily Text, \emph{Text}, \textbf{Text}, \textit{Text}, \textsl{Text}, \texttt{Text}, \textsc{Text}\\
\noindent
\normalfont TT-Family: Standard, emph, textbf, textit, textsl, texttt, textsc\\
\ttfamily Text, \emph{Text}, \textbf{Text}, \textit{Text}, \textsl{Text}, \texttt{Text}, "`Not available"'
\end{document}
Best Answer
You can use any system font with lualatex or xelatex.