will give you the id of the autotile-set. If it’s zero, then it should be the first entry in the list of tiles when selecting your GrassTileMap-node.
To get the id of the subtile instead, you have to use:
GrassTileMap.get_cell_autotile_coord(x, y)
It will return a Vector2, where (0, 0) equals the subtile in the top-left of your autotile-texture, (1, 0) will be the one to the right and (0, 1) the one below it.