This week's book giveaways are in the Jython/Python and Object-Oriented programming forums. We're giving away four copies each of Machine Learning for Business: Using Amazon SageMaker and Jupyter and Object Design Style Guide and have the authors on-line! See this thread and this one for details.
This is a topic that I think needs more stats before it can be answered properly. The Google guys, like on the link Glen points to, say that GWT almost always beats hand written scripts etc, but figures are hard to come across.
As Glen mentions, there is a danger that given the ease of development there is a risk of throwing everything GWT has into your application so code will bloat, not due to GWT but just to using toys that may not have been used if it was more difficult to write.
GWTs approach aims to reduce size of output code as much as possible - each permutation only contains code for a particlar browser; the compiler aggressively removes unused Java code, unused i18n constants/messages are removed etc.