Thank you gifts are not required for family and friends who prepare a funeral luncheon. A hand-written thank you card, however, is proper. The only exception is if the luncheon was a hired event or put together by people who did not know the deceased and then the only obligation is to pay the cost of the luncheon to the person or caterer that was hired.
People who offer to help with a funeral luncheon generally view this as a way to honor the deceased and help the immediate family during a difficult time. It’s a kind and caring gesture that really should only be acknowledged with a thank you note.