I wrote a GPM download script in Python. It doesn't just download full GPM files from FTP, but uses OpenDap to cut out a subset. This should result in much faster downloads and files that don't need postprocessing. The script and a readme can be found here;
https://svn.oss.deltares.nl/repos/openearthtools/trunk/python/applications/hydrotools/hydrotools/datacollection/GPM The script is written to be used operationally, that is scheduled to be run frequently. Every time it runs it will evaluate the available GPM images at the specified URL, check which ones are already downloaded and retrieves only the latest ones. the retrieved files are stored in ArcInfo ASC (not the best, but easy).
In case you want to compile it, have a look here: