I'm not familiar with GWT, but I'm very familiar with Flex. Even though GWT is Java-based (and it sounds like you're largely a Java shop), I'd recommend Flex based on your listed interests:
Futureproof - Flash powers YouTube and a huge percentage of multimedia on the web... they've built it into video game consoles and Intel is building it into settop boxes. It's going to be here for a long, long time, and Adobe has always maintained 100% backwards compatibility with their Flash players.
Works on all major browsers - yes. And with Flash Player 10, that includes Linux systems as well. And with very minimal admin headaches (way less than Java, in my opinion).
Fast & responsive user experience - yes, though Java can be faster in some scenarios. Flash is optimized for vector rendering, so since that's your target, it may be give you better performance than Java.
Code should be unit testable - yes, see FlexUnit.
Code must be maintainable - definitely. AS3 is a serious language, not a toy scripting language. It will be very familiar to users of C# or Java.
Speed & ease of development - for what you're doing, you'll be able to find dozens of examples of exactly what you described: zooming and panning Google Maps -like vector images. The core of the runtime is a well-designed graphics engine, so building an interactive graphics app is its natural chore.
Supports vector graphics of some sort (SVG a plus) - obviously, Flash does this natively. SVG is supported at the compilation stage (you can compile in SVG but you can't parse SVG at run-time). There are toolchains available that'll convert SVG to SWF for you at runtime.
The only reason I'd see for using GWT would be to keep your shop Java on server and client... but then again, you're looking for the best tech for the job, right? In that case, I'd say Flash (and in particular, the Flex framework) is best suited for what you're wanting to accomplish.