Gran Canaria airport now has a small but well stocked Spar supermarket on the ground floor by the arrivals lounge. Prices are reasonable and you can buy all the basics like bread, milk, fruit, drinks, toiletries etc. 

The Spar is open every day from 08.00 until 22.00 so will be be useful to almost everyone who arives at the airport. If you arrive in Gran Canaria after 21.00, then it's worth getting anything you need for the night or first thing in the morning at the airport Spar as most resort shops close at 22.00.