GWT is a different cup of java. Not for php "Joomla" "Drupal" webbuilders. No offense.
Just want to warn you and your boss.
Building a GWT Site needs more skilled programmers who know Java.
You also need to think about how you want to host it. If you are limited to Apache2 or Apache webserver then you are limited in what parts of GWT you can use.
In GWT you can have a pure script site. This would not be interacting with servlets. However as I said that is limited. Using GWT extensively you will not be able to avoid writing servlets.
If that is a problem for you or your manager doesn't want to spend the money it costs to actually build it. I have recently met such a situation where staff thinks GWT is going to cut production time to 10% but that is simply not true.
GWT is an alternative to for example
jsp or velocity. Yes GWT is a framework but not like Joomla or Drupal is a framework.
There are plugins for eclipse wtih wich you could do a lot of visual drag and drop but that is clientside. You still need to write server side code.
And don't forget about login and such..... You would have to take full controle of coding permissions into your code again unlike working with a thing like Joomla or Drupal
GWT is great stuff but you need some java knowledge.