Debes agregar una zona donde se repitan los pixels alrededor de cada tile.
https://www.yoyogames.com/blog/3/seamless-tile-scaling-in-gamemaker
http://gmc.yoyogames.com/index.php?showtopic=546371
https://www.yoyogames.com/blog/3/seamless-tile-scaling-in-gamemaker
http://gmc.yoyogames.com/index.php?showtopic=546371