There is inventory level integration between Dynamics GP and Dynamics RMS.
There is a free option directly from Microsoft for GP to RMS Headquarters:
Professional Advantage makes a GP to RMS integration that several GP Partners have recommended to me. (but I've not needed to use at this point yet)
For Dynamics GP to Dynamics RMS Store Operations it is a 3rd Party ISV Product:
For those of you who have upgraded from Microsoft Small Business Financials this is just one of those many new things you can do that you couldn't before!