It should work after you click on setup autotile, so long as the autotile script is set, I use it myself so it definitely works. By default the autotile script should be the one at res://addons/fabianlc_gpu_tilemap/autotile scripts/default_autotile.gd, if the autotile script property of the gputilemap node doesn't have anything for some reason it can't work, if you really think there's something broken you can open an issue on github and send a project where you can reproduce the issue.