The problem is that GWT bundles the 32-bit version of SWT, which it requires in order to run the hosted browser. So, you can install the plugin under a 64-bit version of Eclipse. However, you'll have to use a 32-bit JDK for your GWT projects in Eclipse.
The next version of GWT (or, if you build GWT from the trunk) supports Out-of-Process-Hosted-Mode (OOPHM). This allows you to debug your application against a real browser running on the platform, instead of GWT's hosted browser. As a result, GWT no longer needs to bundle SWT, so you'll be able to use a 64-bit JDK with your GWT project.