Not really. The majority of the Gamelan parts are written down and learned by rote. Effectively they are repeated loops. The Bonangs- that's the things like inverted cooking pots- improvise around the framework of the chimes, the improvisation is rhythmic as well as melodic. If it is used, the Rebab- a two stringed fiddle, will also improvise. The Drummer effectively controls the performance, like a conductor, and signals the changes from one pattern to the next in a similar way to the leader of a Samba batteria uses a whistle.
Cheers
Dave