Set aside implementation details for a second; what are you actually
trying to accomplish?
The short answer, to respond to the question as you've phrased it
here, is that it's impossible(*) to write a browser-based application
(with GWT, hand-crafted JavaScript, or any toolkit) that will
programmatically retrieve a list of files on the client. Browser
security rules forbid anything like that (for obvious reasons -- it
would be Very Not Good if any random web page had the ability to go
snooping around in your filesystem).
(*) In this context, "impossible" disregards messing around with local
security policy files and/or using browser-specific hacks.