Before you start using Google WTP i strongly sugest that you read this post by Jevgeni Kabanov. Do also check out the alternative projets the post refers to! There are lots of alternatives out there:)
Opinion: Will Google Web Toolkit  Matter?
I’ve listed some of the projects here if you don’t bother to read the post(but you should):
- Java2Script:
 Converts java to java-script (Has plugin for eclipse and uses Eclipse SWT libraries for the graphical elements)
- DoJo:
 Provides a JavaScript Widget toolkit
- Echo2:
 “a platform for developing web-based applications that approach the capabilities of rich clients”
- DWR:
 “DWR is easy AJAX for Java.”