I like a lot how some Google applications are making use of Gears. A couple of their applications like: Google's Docs family (Documents, Spreadsheets, Presentations, PDFs), & Google's reader, I'm not sure if there're other services that are using Gears, but I use those 2 heavily.
The question is: why hasn't Google used Gears with GMail yet? Imagine how it would look like.. Specially, that it goes with Google's effort to make GMail widely accessibility. Check -for eg- GMail for mobile phones (use it.. it's hilarious) which support caching & other stuff that might be provided by Gears on PCs.
UPDATE: (3 hours after publishing this post, the first time)
I've just realized that Google made my wishes comes true a couple of days ago through a new "labs" feature :D.. Though I've been dreaming of this since I knew about Gears :D.. Read more about offline GMail here. I'm laughing & rolling on the floor right now!! looooooool!!
Now, I'm waiting for Google Calendar, to use Gears..:D