Gravity Forms Block Email Domains is an add-on plugin for Gravity Forms that allows you to define a comma separated list of email domains to block on each email field. Custom validation messages can be set as well. Blocked email domains will prevent the form from being submitted and prompt the user to enter a non-blocked email address.
Visit Plugins > Add New in the WordPress admin dashboard
Search for “Gravity Forms Block Email Domains”
Install and then Activate Gravity Forms Block Email Domains from your Plugins page
Navigate to your gravity form
Set a comma separated list of blocked domains and a custom validation message on an email field in the advanced tab