You can find documentation for the various packages in the Gutenberg Github repo in the readme for each package.
For example, the "data" module docs are here: https://github.com/WordPress/gutenberg/tree/master/packages/data
I will search a term in the Gutenberg repo, and show just the Markdown files to quickly find any related documentation.
As far as seeing what objects or functions are available for any given package, I use the browser console to explore things. Open an admin page with the Block editor, open your console, and type "wp."
It will show you a list of all the apis. as you select each object, you can then type another "." to see the methods or properties it contains. This is really helpful in understanding how things work and what methods are available.
I'm often looking around in
wp.data.select( 'core/block-editor' ) for methods to use when building my blocks. So I type
wp.data.select( 'core/block-editor' ). in the console, and it shows a list of everything that's available. If I need to know more about a method, I'll then search the gutenberg repo for it to find more details.
Hope that helps!