Besides the examples mentioned by others, gradients can arise due to look angle. For instance, looking upon a field of mixed grass and dirt from straight above

Besides the examples mentioned by others, gradients can arise due to look angle. For instance, looking upon a field of mixed grass and dirt from straight above versus 10 feet towards the horizon will yield a gradient from green/brown to totally green. There are lots of similar examples, for example, look at a mixed asphalt/composite road stretching off into the distance.