What are some examples of writing matrices in the Gummi latex editor? Something about the syntax used on the forum doesn't work for me when used in Gummi.
pmatrix is an extension from amslatex. In the preamble of your tex file, add
\usepackage{amsmath}
and it will work. There are many useful extensions in amsmath and I use it in almost all my documents. See the ftp://ftp.ams.org/pub/tex/doc/amsmath/amsldoc.pdf.