Chrome uses WebKit as their engine, which is written in C++. In terms of UI and the remainder of their internals, I believe it is custom-done, also in C++. I haven't looked too in-depth into their code, but you can get a lot more information about the source tree here: http://dev.chromium.org/developers/how-tos/getting-around-th...
The Google Web Toolkit doesn't really make sense for a browser, it's designed to make websites.
Chrome is written in C++. Since it's open source you can even check out the code: