Angela,
Spain used to be a difficult country to drive around away from the main cities and Mediterranean coastal belt. But in recent years this has all changed. Massive investment has resulted in huge upgrading of most rural and cross-country routes. So driving and hiking in rural Spain is now a pleasure, with good traffic free main roads through spectacular scenery. Motorways - autovia - incur a toll, while dual carriageways - autopista - are often, but not always, free. Sign-posting is good except in towns, where you need to keep a good look-out for small signs often on the far side of junctions.
In Spain the Federación española de deportes de montaña y escalada is responsible for the maintenance of the GR maps. Many GR routes make up part of the longer European walking routes which cross several countries. Thus, there's a concerted effort to insure they are as complete as possible.
As always, when there appears a question of "Go right or Left" ask a local. Unless your a man...follow your nose, go the wrong way and...when you finally admit you're wrong...Ask for directions!
Buen Camino,
Arn