Are you looking for user training or developer training?

You can order/download free demo from following website http://www.microsoft.com/dynamics/gp/product/trial.mspx, which work beyond 90 days for training purpose.

You can learn lots of things just playing with it and reading help topics. Any questions you can ask on GP forum:

http://www.microsoft.com/Businesssolutions/Community/Newsgroups/dgbrowser/en-us/default.mspx?dg=microsoft.public.greatplains&lang=en&cr=US