\documentclass[a4paper]{article}
%\VignetteIndexEntry{gamselBayes User Manual}
%\VignettePackage{gamselBayes} 
\usepackage{pdfpages}

\begin{document}

\newread\PDFtoInclude
\openin\PDFtoInclude=gamselBayes.pdf
\ifeof\PDFtoInclude
\typeout{No file gamselBayes.pdf}
\else
\includepdf[fitpaper=true,pages=-]{gamselBayes.pdf}
\fi

\end{document}