I think Positive is just a better description of gentle. A lot of time, a broad term for things can, over time, can get polluted with compromise...
for example (an extreme one to make a point) : Some one could label making a child sit in a corner facing the wall for a half an hour - for coloring on the wall - a "gentle" discipline bc it's not violent or yelling.
Positive discipline sees that that is still very damaging to a childs spirit and does not teach them in a complete way... Positive discipline seeks to build up the child's character while teaching them valuable lessons... Like, now the child has to stop his play to clean the wall. (This being done in a way that encourages the child to continue being artistic but letting him know the boundaries...)
I hope that made sense.