With command \resizebox{new width}{new height}{what you want to resize} you can change dimensions of any latex box. This means that everything (text, lines, simbols, ...) will be adjusted to new dimensions. In first or second parameter you can use ! in order to keep original aspect ratio. With this command,
\documentclass{article}
\usepackage{lipsum}
\usepackage{pgfgantt}
\begin{document}
\lipsum[1]
\noindent\resizebox{\textwidth}{!}{
\begin{tikzpicture}[x=.5cm, y=1cm]
\begin{ganttchart}%
[vgrid, hgrid]{50} % 50 weeks
\gantttitle{Title}{12} \\
\ganttbar{Task 1}{1}{4} \\
\ganttbar{Task 2}{5}{6} \\
\ganttmilestone{M 1}{6} \\
\ganttbar{Task 3}{7}{11}
%\ganttlink{4}{2}{5}{3}
%\ganttlink[b-m]{6}{3}{6}{4}
%\ganttlink[m-b]{6}{4}{7}{5}
\end{ganttchart}
\end{tikzpicture}
}
\lipsum[2]
\end{document}
your Gantt chart would look something like:

Update: Solution for pgfgantt 4.0
As is stated in Missing number treated as zero in gantttitle, last version of pgfgantt is not completely backwards compatible. And now ganttchar needs two mandatory arguments
\begin{ganttchar}[options]{<start tss>}{<end tss>}
Therefore, previous example should be:
\documentclass{article}
\usepackage{lipsum}
\usepackage{pgfgantt}
\begin{document}
\lipsum[1]
\noindent\resizebox{\textwidth}{!}{
\begin{tikzpicture}[x=.5cm, y=1cm]
\begin{ganttchart}[vgrid, hgrid]{1}{50} % 50 weeks
\gantttitle{Title}{12} \\
\ganttbar{Task 1}{1}{4} \\
\ganttbar{Task 2}{5}{6} \\
\ganttmilestone{M 1}{6} \\
\ganttbar{Task 3}{7}{11}
%\ganttlink{4}{2}{5}{3}
%\ganttlink[b-m]{6}{3}{6}{4}
%\ganttlink[m-b]{6}{4}{7}{5}
\end{ganttchart}
\end{tikzpicture}
}
\lipsum[2]
\end{document}
\begin{document}? – Jake Jun 08 '12 at 07:05