I need to create javascript library which will be called from other javascript code.
Is it possible to create javascript library using GWT and Java classes? No entry
Yes, it's possible to write GWT code that other JavaScript on the page can call.
However, I don't know of any projects that currently do this in any large scale, so you maybe blazing a new trail.
The basics of calling GWT code from JS is covered in this GWT blog post from 2008, particularly the section called "Creating JavaScript libraries with GWT" unsurprisingly.
Another project which is linked in that blog post, and maybe of interest to you, is GWT Exporter, which "contains a generator capable of taking GWT classes and exporting them as ordinary Javascript APIs callable from hand-written Javascript without JSNI"