Where is the list, or gallery of the patterns available for pgfplots?
The pattern list itself is not directly related to pgfplots, but to tikz and pgf. The pattern list is available at section 26 "Pattern Library", of "TikZ and pgf Manual".
Or at page 217 of:http://paws.wcu.edu/tsfoguel/tikzpgfmanual.pdf
The current list of patterns listed at manual version 1.18: