How to change to this graphic style?
If you use an external map editor, like Tiled Map Editor (which most people who make custom maps use), you can use a virtually unlimited number of other styles you can find online. All that matters is that the size is 32x32, so that it lines up with Gather’s grid.
Here’s a list list of tilesets that are compatible with Gather. There are lots more on that website, too.