Restaurants in San Sebastian
Gateau Basque is far, far more readily found across the border in the French Basque Country, Pays Basque. To find it in San Sebastián you would need to go to a very old time traditional Basque restaurant rather than the nueva cocina restaurants, but I honestly can't ever remember seeing it on the menu. If on a menu, it would be called "pastel vasco" in Spanish.
But if you cross the border and go to the market in St-Jean-d- Luz or Biarritz you can find the stand of either the Hotel Arraya of Sare or the Moulin de Bassilour in Bidart, which both sell a phenomenal gateau basque. We have been to the demonstration of pastry chef Bixente Marichular at the Musee de Gateau Basque in Sare. www.legateaubasque.com