Your question is two-fold; first is food a supply or service and second can the GPC be used to purchase it.

First, according to Army Regulation 750-2 “Supplies include food, clothing, equipment, arms, ammunition, fuel materials, and machinery of all kinds.”  In fact, Food and Subsistance items are catorgorized as Class 1 within the 10 classes of supply.   In the situation you stated, you are purchasing the food and the catering (service) is incidental to the supply.

 The second part of the question is covered by the DoD Government Purchase Card regulation specifically prohibits the purchase of food.  “Food and Meals. Generally, use of the GPC to purchase food and meals is prohibited. However, if an agency determines that a reception with “light” refreshments (no meals), as provided in the Office of Personnel Management (OPM) Federal Personnel Manual, would materially enhance the effectiveness of an awards ceremony conducted under authority of the A-17 Unique Business Rules for Purchase Card Programs Government Employees’ Incentive Awards Act,15 the cost of those refreshments may be considered a necessary expense for the purposes of the Act. Written coordination with local legal counsel and the Finan15 See 5 U.S.C. 4501-5606. A-18 Unique Business Rules for Purchase Card Programs cial/Resource Manager is required prior to purchase of any such refreshments.