The MRP functionality within Dynamics GP is very powerful. It looks at a number of supply and demand factors in order to arrive at its results.
The Sales Forecasting module is the best way to manage forecasted demand within the system. You can specify by product and time period, what the forecasted sales quantity is. You can combine several forecasts to roll up quantities to the division or organization level.
Here is an example of the Sales Forecasting screen in Dynamics GP:
Here is a quick video that shows using sales forecasting to generate MRP results.