It's easy, and I'm doing it from GMAIL right now!
In a nutshell; under gmail SETTINGS, enable "add gadget by URL" and then save. You then go back into SETTINGS and there should be a GADGET tab. Select it, and enter this URL - http://www.blogger.com/gadgets/post.xml. Bob's your uncle :)
Go to http://www.google.com/support/blogger/bin/answer.py?answer=143415 for all the details.