WPComment Blacklist support for Gravity Forms
WordPress Comment Blacklist support for Gravity Forms adds WordPress default Comment Blacklist support to active gravity forms. You can select forms from the plugin setting page. Please, see the screenshot-1 for settings guide.
Install and activate the plugin from your WordPress Administration panel
Go to Dashboard > GF Blacklist Settings, select forms and hit the “Save Changes” Button.
Can I customize validation message?
Not yet but in the next update.