You can customize the default values populated into the Google Analytics variables by following these steps:

1) Go to Account → Account Settings

2) Edit the value for the following Google Analytics parameters:

• Default UTM Source: The value that will auto populate in the "Source" field on send screens when Google Analytics is enabled.
• Default UTM Medium: The value that will auto populate in the "Medium" field on send screens when Google Analytics is enabled.
• Default UTM Content: The value that will auto populate in the "Content" field on send screens when Google Analytics is enabled.

3) Once these settings are set, you will see these values populated as the default values when you enable Google Analytics for a send.