There is documentation of GeometryDashAPI

See the parts you are interested in on the pages

Game saves — if you want to load .dat files
Levels — if you want to manipulate with game objects (block) in the editor
Network — if you want to communicate with some game servers
Resources — if you want to load some from GeometryDash/Resources directory, e.g. official levels
Serialization — if you want to know how library works, the page can be useful if you want to create your own objects
Testing — if you want to know how we test this library