Download the GWT plugin  zip file entirely and unzip in the plugins folder of Eclipse. There might be a few more instructions that you should have to follow other than just unzipping the package, so double check with the GWT site before you do.

Either way, last I read that the GWT plugin was real buggy and kept crashing Eclipse while repeatedly trying to update some resources which took toll of the CPU. This also happened to me during a demo! Research this out before you give it try. This was about 2-3 months ago, which GWT confirmed the bug but I dont know if they have put out a fix for it yet.