It´s a weird problem as horizontally the tiles are good, but vertically they show a 1 or 2px gap. Everything else is scaled correctly. This looks like a very obvious bug in GM, I mean, I´m just using basic stuff.
I guess I´ll have to use surfaces then, Thanks!
BTW I didn´t know now GM draws by default to a surface, that´ll save a lot of work.
I remember having this problem back when I used pixel art graphics, I think there's a solution here: https://www.yoyogames.com/tech_blog/8
@SquirrelWide nice triple post man, use the "edit" button xD