It is possible to sell items without setting them up in Dynamics GP. The setting in Dynamics GP required to enable this feature is maintained in Sales Order Processing Setup, but first you have to make sure you have not enabled the Add Item Option at the Sales Transaction Entry Window.
Under Tools > Setup > Sales > Sales Order Processing there is an Options Button, which opens the Sales Order Processing Options Setup window. In the lower portion of this window there is a flag which allows for the entry of non-inventory items, which must be checked to process as you desire:
Mark this option to enter non-inventoried items in the Sales Transaction Entry window for a sale or return without creating records for those items first. If you mark this option, any item numbers entered in the Sales Transaction Entry window that don't exist will be recognized as non-inventoried items. If you don't mark this option, only items that exist in your inventory can be entered.
If you are prompted to add items, which are not already configured in Inventory Control, then one or the other of these settings are not set correctly. Finally, in order for accounting distributions to be correctly assigned to transactions of non-inventory items, you must choose a default account in the Posting Account Setup window. Navigation: Microsoft Dynamics GP >> Tools >> Setup >> Posting >> Posting Accounts.
Failure to assign a default posting account will result in transactions not posting properly.