There are some macros in init_constants regarding tiles. Update TILES_PER_ROW, TILES_WALKABLE_COLUMNS and TILES_SOLID_COLUMNS to match the tileset and everything should work.
Keep in mind, changing the width of a tileset will change tile IDs in Game Maker (since it's linear) so any rooms using that tileset will be corrupted. After you use a tileset once, you should only change the height if possible.
