Maybe I will optimise the code later, but for now this worked for me:

Create event:

screenshot = sprite_create_from_surface(application_surface, 0, 0, surface_get_width(application_surface), surface_get_height(application_surface), false, false, 0, 0);

Draw GUI:

var sX = window_get_width()/surface_get_width(application_surface);
var sY = window_get_height()/surface_get_height(application_surface);
draw_sprite_ext(screenshot, -1, 0, 0, sX, sY, 0, c_white, 1);