I spend alot of my time in Emacs, and move between Windows, Linux, and Mac OS at least once a day since these are the machines my company has available to use. I spend alot of time in gmail using gmail folders too, and would like to find a cross platform email client for emacs which can support gmail too.
Note: I would like to find a client which is "stable", and has good support and documentation.
Best Answer
A long time ago, I used to read my e-mail in gnus. A quick search shows that gnus supports IMAP, and since GMail supports IMAP, it should (in theory) work. Here's the some documentation on how to setup imap in gnus: http://www.gnu.org/software/emacs/manual/html_node/gnus/IMAP.html.
I will also add that I have not tried IMAP in gnus myself, so YMMV.