Guyfawkes has updated his GP2x User Guide which is the most comprehensive information for starters. Release notes:

I have released a new version of the GP2X User Guide. This update is mostly minor changes to various parts of the guide and an update to the GP2X dev introduction which should be more complete now (still not finished though) as I forgot to mention you need MinSys installed. Thanks to everyone that helped out and emailed me additions or changes, I have added you to the Thanks page in the guide. In the next few days I will be giving the guide to a few people who will translate the guide to German and French, if you would like to translate to another language please let me know at guyfawkes@NOSPAMgbaemu.com or PM me.

http://archive.gp2x.de/cgi-bin/cfiles.cgi?0,0,0,0,23,933