You can create your deployment password in the Cloud Console .
Go to cloud.google.com/console , select your project and in the menu list on the left, expand cloud development and click "Push-to" -Deployment.
An alternative way to access the App Engine console is in the Administration section in the Application Settings section in the Deployment Path section by clicking here.
I assume you already know this since you have successfully set up the repo.
Now that you are in the Push-to-deploy settings, you will see the Get password and Delete repository buttons at the top. Obviously you need the first one.

You need to authenticate with Google OAuth, select the account that is the developer account in your application, and click Accept to provide read and write access to the Google Code .
After granting access, you will see a page with the message:
This is your git password:
...
To use this password, add the following text to the .netrc :
machine code.google.com login you@gmail.com password yourpassword
Learn more about using git and Push-to-Deploy.
You need to copy the code to a .netrc , which should be in your home directory. If it does not exist, just create it.
On Windows:
Like eddard.stark mentioned in his comment, in windows, the .netrc should be named _netrc .