I haven't personally used GWT, so all of my information is based on what I have read.
Laszlo is a declarative language much like XHTML, but it also supports classes. This allows developers to quickly lay out the skeleton for an application. This is such a very powerful tool for creating graphic applications, but I consider it a *must-have* feature for any language intended for creating web applications. Laszlo was released before Flex, and the Adobe developers saw Laszlo's declarative nature and decided to make it a central part of Flex. But I don't believe that GWT supports declarative markup.
But even more important is that a Laszlo application isn't tied to any particular platform. Laszlo was designed to be relatively easy to port to different web-based platforms, so a single set of Laszlo source code can be executed on Flash, DHTML, J2ME and other web-based platforms that might appear in the future. It is my understanding that GWT is pretty much wed to DHTML and Google doesn't have any plans to support other platforms in the future.