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