For this map I used a photoshop plugin, but yes basically the way it works it is just a bunch of custom brushes that lets you paint, paths, grasses, rivers, hils, etc...

The reason I recommended gimp to you is because that is the closest thing you can get to photoshop that is free, so if you take some time to learn it you will be able to create maps that look just as good. There are tons of tutorial videos on GIMP on youtube so if you ever get stuck you can always find a tutorial video

There might even be some custom plugins for GIMP for map creations like this, but I dont know I have not looked into it.