It should not be hard for a static webapp, since GWT just makes some JS/CSS/HTML files. So if you can access files in this manner on Sharepoint, it might work.
However if you want RPCs and server side actions, you'll have to see what kind of code you can run on Sharepoint, which is probably limited to .Net, if anything. I would not expect a Java backend to work on Sharepoint without substantial effort.
Sincerely,
Joseph