This is a great pause menu but i found that if you have a view port the menu would be centred. I would recommend:

pause = false;
gwidth = view_get_wport(0);
gheight = view_get_hport(0);
cx = gwidth/2;
cy = gheight/2;
screen_alpha = 0;
screen_alpha_set = 0;

instead of 

pause = false;
gwidth = display_get_gui_width();
gheight = display_get_gui_height();
cx = gwidth/2;
cy = gheight/2;
screen_alpha = 0;
screen_alpha_set = 0;

but apart from that this is the best pause menu :)