Many people use pencil lines on the riser to create distances for the GAP, when learning, either as a second reference or the arrow tip or as the primary aiming, or a combination of both. They are not legal in competitions BUT, with the pencil lines on the riser, once you get the GAPS memorized, you will be able to shoot more accurately without them.
As already mentioned, you can use a simple matchstick/rubber band (or just the band alone) arrangement.