Hi tog, Doug from basqueMTB here! We're based outside Donostia (aka San Sebastian). From Gasteiz (aka Vitoria) it's about an hour to us and the riding we have. From Gasteiz it's also an hour to Iruña (aka Pamplona) and there's riding around there although it's not a place we'd normally ride since we have plenty of trails closer to us.
There's some info on our website although we're currently getting a flashy new version done and it's a bit behind the times! There's loads of pictures and videos of where we ride on BASQUEMTB – FLICKRif you want to check out the type of thing we ride.
It's worth saying to you though that there's some Spanish 'trail centres' around that area which the government has put on the web and given GPS routes for. They're great. If you like 100km's on fireroad 🙁
Give me a shout, either here or on doug@basqueMTB.com if I can be of more help.
(Hi GW, thanks for putting up the info!)