GWT to create utility javascript library
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"
Check out:
https://code.google.com/p/gwt-exporter/
Develop an application or library in GWT and use gwtexporter annotations to make classes and methods available from javascript.