I walked in Gran Canaria following the walk routes of sunflower book. I don't know about GR131. I never had prob with authority. The biggest problem was the dogs. They barked and yapped from garden, roof top, on the street…. Some were loose. A little one tried to nip my ankle. Fortunately my boot was too tough for him. Local people don't walk, only foreigners do. Footpaths are not well defined and I could not find any good map like OS. I met a walking group from Europe, who spent winter months there. And so, I went with them from then on. We often asked locals for directions. They were too pleased to see us in remote areas and very chatty. ..that was…20 years ago.