Hi, thanks for using my asset! If you want to be able to resize your game window, you’ll need to resize the GUI surface; GameMaker doesn’t do this for you except at the start of the game. In either a step event or the resize event, you’ll just need to check if the window dimensions are different from the GUI surface size, and if so, call display_set_gui_size(), and grab the window dimensions with window_get_width() and window_get_height().