There is some information on this page and you can read that page too if you like the zoo-like ambiance of finding a few gems of information amid all the monkey screaming.
Succinctly, for access to Gmail specifically, the App uses the password upon the first connection, to obtain a specific token value; think of it as a randomly generated sub-password. The token value is enough to read incoming emails, send emails, and alter the mailbox. It does not, however, give any power beyond these operations, so knowing the token value does not give you access to the actual password or to other Google sites (like Google+). The App stores the token value, not the user password.
So this answers your question: if someone steals or subverts your smartphone, he can obtain the token value which grants access to your emails, but he won't be able to recover or reset your password, or access any other service linked with your password. Of course, a few caveats apply:
Someone who controls your emails can then leverage that to attack all systems who use an email-based password-recovery system. A lot of sites implement a "forgotten password" feature in which a password-reset token (say, URL) is sent through email. Gaining access to your emails is kind of equivalent to stealing your Internet life altogether.
An attacker who could subvert your phone enough to read the private data storage of the Gmail App is most probably able to plant a key/screen logger and obtain your actual password the next time you type it.