If you want to default to typing in your pw each time you access your email you can try this.
Go to the Utilities Folder inside the Applications Folder.
Open Keychain App
Search for your gmail account in the list
or put gmail in the search box & search.
When you find it, double click the line in the list....
then double click the show password in the dialog box, which will need your computer password to allow you to see this.
If this is the correct one then go back to the line you chose in the list and hit the delete key.
The entry for gmail will be gone and next time you log in for your email you will be prompted for pw for gmail.
Just don't click the "Remember my Password" when it asks!
Now that you know where all your pw's are kept... make sure your computer has a simple secure overall pw.
A lot of people are becoming aware that anything connected with google is monitored, collected and available to the internet.
G Search, gmail, FBook etc is being monitored at least by the NSA if not also cracked by others.
Aviator.app & TorBrowser.app are a secure Free browsers for the Mac... if you need it.
Rethink what you are using for a password
Using an Email address for a username is pretty easy to figure.
So make you password stronger by making a silly sentence with the odd Uppercase character & a number in the middle.
Check the strength here:
The WhiteHat Aviator Web Browser
Storing all your Passwords & Personal Info
SplashID Password Manager