Had you used mutt instead of Gmail, you would have simply used: SHIFT-zSHIFT-z. When you want to compose mail in one of these programs, it opens your specified text editor and uses the whatever text you enter as the contents of your message. It works a lot like the 'Intents' feature which has been Android has received so much praise for, only it's decades old.
Actually, there's no reason for this to depend on the cli. In Firefox, there are plugins which give you a button to click under every text box which open your favorite text editor with a buffer already open containing the contents of the text box. Edit the buffer and save, and your changed appear in the text box. (This is, in fact, how I am editing this comment.) On Unix systems, a graphical mail program could easily use a shell call to get text from an external program. The only problem is, this just doesn't really happen. Vendors prefer proprietary interop with their own products rather than more universal interop methods.
Actually, there's no reason for this to depend on the cli. In Firefox, there are plugins which give you a button to click under every text box which open your favorite text editor with a buffer already open containing the contents of the text box. Edit the buffer and save, and your changed appear in the text box. (This is, in fact, how I am editing this comment.) On Unix systems, a graphical mail program could easily use a shell call to get text from an external program. The only problem is, this just doesn't really happen. Vendors prefer proprietary interop with their own products rather than more universal interop methods.