I've checked the library here: charlierguo/gmail · GitHub. Did you check this site as well? According to the description in there, in order to get all the messages in your inbox you have to use:
In your code, you are printing the value of "t" which it should be True because you are logged in. The description explains that you can use g.logged_in to check if you are logged in (charlierguo/gmail · GitHub). It will return True is you are. So that is the True value that you get in your print statement.
You have print the value returned by g.inbox().mail(), so you could modify a little bit your code by adding the following lines:
messages = g.inbox().mail() print messages
It should print the output of g.inbox().mail() which according to the description, it should be the messages in your inbox (charlierguo/gmail · GitHub).