From DJ's site

Braindump: SetUpEclipseForGWTAndGXT

To develop nice Web GUIs with Ext-GWT (abbreviated GXT) using the Eclipse IDE, do the following:

• Download and install the Eclipse IDE
• Download Google Web Toolkit and extract the contents to your hard drive
• Download Ext-GWT and extract the contents to your hard drive

The versions I used when writing this are:

• Eclipse 3.5 ("Galileo")
• GWT 2.0.3
• GXT 2.1.1

Now, let's add User Libraries for your convenience:

• Start Eclipse
• Open the preferences dialog (Menu: Window -> Preferences)
• In the tree on the left navigate to item Java -> Build Path -> User Libraries
• Add a user library for GWT
  • Click button "New..."
  • Name the new user library "GWT_2_0_3"
  • Select "GWT_2_0_3" from the list and click button "Add JARs..."
  • Navigate to the place where you extracted the GWT zip file and select the files gwt-user.jar and gwt-dev.jar.
  • Click button "Open"
• Add a user library for GXT
  • Click button "New..."
  • Name the new user library "GXT_2_1_1"
  • Select "GXT_2_1_1" from the list and click button "Add JARs..."
  • Navigate to the place where you extracted the GXT zip file and select the file gxt.jar
• Click button "Open"

There you go. That's all there is to set up Eclipse for GWT and GXT. Now have a look at Setting up a GXT project in Eclipse.