Oh, as a side note, is this shader compatable with DrawGUI? in the sense that things drawn in DrawGUI can go under the shader?
If you want GUI elements to be a part of the CRT, do this in any object's Post-Draw event. Just note that if you have an HD HUD over a pixel art game, it's gonna look weird. This works best for pixel HUDs at native game resolution. If you're using a view surface instead of the application surface as your CRT input, just add a reference to it as a parameter for crt_gui_begin().
crt_gui_begin();
draw_text(10, 10, "\n\nTest GUI");
crt_gui_end();