I have a game that has a 4:3 aspect ratio (640 x 480 resolution) and I wanted to make a custom border like how they would do it in Nuclear throne or the console version of Undertale. How do I achieve this?
You have two options:
1 - Use views and cameras;
2 - Draw the borders on the GUI, you can scale the GUI beyond the game viewing area. display_set_gui_maximize()
You can choose the one that is most suitable for your game. I consider the second option the easiest, however I recommend the first option if you have more skill with that.
But is it really necessary to draw that border?