I don't know of anything good in english on the internet, I am sure there is some, I just kind of learn this stuff at rehearsal in the group I am in. There is also this book [1] if you are really curious (its a beast of a book).

But to briefly explain what I am thinking, there is the notion of irama [2], where initially (in irama I) most instruments play something close to what is written. At some point (depending on the piece and style, but usually signaled by the drummer in Javanese gamelan) there is a transition to irama II, which sounds like everyone slowing down at first, but as soon as you get to some terminal point of the transition, certain instruments, based on their own rules, begin to "fill in" the space created by the transition. For example, if you are playing the peking, it might look like this:

main melody/irama I/balungan: 2 1 2 6

peking in irama I: 22 11 22 66

peking in irama II: 22 11 22 11 22 66 22 66

and so on

I am far from expert and so much of this is all totally different depending on region and such. But this core idea of irama and rules-based elaboration from a specific seed is somewhat always present.

Either way, just really some of the most beautiful music you will hear, highly recommend to everyone [3].

1. https://www.amazon.com/Gamelan-Manual-players-central-Javane...

2. https://en.wikipedia.org/wiki/Irama

3. https://youtu.be/3efH3WgTQRU