I ran across this post from Mohammad Daoud recently. I get requests all the time from clients needing to have Dynamics GP transactions posted automatically. Mohammad did some digging and found a tool from Microsoft developed using the Continuum API to post GP Transactions. GP must be opened and a user must be logged in but that's generally not too much to ask.
You can use it to post GL, Inventory, Cash Receipt, Sales Order Processing, and Receiving transactions. It's not free but most great things aren't. Check out Mohammad's post for more details http://mohdaoud.blogspot.com/2010/07/posting-microsoft-dynamics-gp.html.