Yeah, there's a reason why you find no special code for this in the player object: it's actually the grass that handles it. Basically, the grass is in front of the player normally, but each grass object draws the top of the player after drawing itself if the player currently is colliding with it. (Basically, I use the "painter's algorithm" to make part of the player be above the grass... by just drawing it on top of the grass)
(For some reason I use dynamic rendering masks for this - I just realized that just drawing the upper half of the player's sprite with draw_sprite_part would've probably done the same thing but with much simpler code... x3)