I recently used Gaia GPS ($20ish on iOS) on a few Utah hikes. It was useful but I don't know that I'd want to be completely reliant on it. The cacheable maps it was using in The Maze district of Canyonlands, for example, were missing some trails and features. When the GPS was working (outside narrow canyons), I found the topography and accuracy quite good, but even basic routes were creative enough that you needed to know where the trail was or you wouldn't make it through.