A little known feature of Dynamics GP is extended pricing. Most customers believe that by default it is enabled, but in fact the price levels that we look at are standard GP pricing. Extended pricing allows you to do many things including using Price Books to create promotional time sensitive sales and assign pricing to a specific customer as opposed to a customer level.
To enable this feature in GP, navigate to Tools -> Setup -> Sales -> Extended Pricing. This screen is very simple and simply allows you to enable or disable extended pricing.
Once this is enabled, you must get everyone out of GP and restart your clients. At this point, you will see a new Extended Pricing tab under the inventory screen. There is an advanced flow to how these are assigned, see below for a basic diagram to get an idea of how to setup your customers.