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Those lines are caused by the way the graphics card interpolates the texels from the texture. Increase the texture border size in the tileset editor for the tileset from 2 to 3 or even 4 and see if that helps. To explain why this happens, have a read at this tech blog (it's for GMS1.4 but equally valid for GMS2): https://www.yoyogames.com/blog/3/seamless-tile-scaling-in-gamemaker