I want to have a look in other aiming methods and tried to start off with "gap shooting". However, I'm not quite sure whether to leave both eyes open or one eye closed. What is the right thing to do?
When gap shooting or shooting a bow in general, there are a couple of advantages to keeping both eyes open. One is that having both eyes open gives you some amount of depth perception, so you can see how far your target is from you. Another is that with both eyes open there's less tension in your face, so your whole body can be more relaxed, and that should help you aim better.
According to the Archer's Bible,
"A type of sighting akin to the instinctive method, one that makes use of a "space picture," is called "gap shooting." In this method the archer looks at the target and nothing else but is still conscious of seeing the amount of space between the arrow tip and the target. The size of this space or gap becomes a measure of the angle of elevation, and the archer either consciously or unconsciously uses this measure for a distance."