A Mailchimp API key is required for Gravity Forms to automatically add people to your Mailchimp subscriber list.
Create a Mailchimp API key by logging into your Mailchimp account and then navigating to your profile, Extras, then to API keys.
If you don't already have an API key, click Create A Key to create one. Copy your API key.
To connect your Mailchimp account to Gravity Forms on your site, go to Forms, then Settings from your WordPress dashboard. Then, select Mailchimp and paste your API key and click Save Settings.