Blocks words in Gravity Forms (WordPress Plugin) based on the disallowed words in Settings > Discussion (where you'd usually block words for use in comments)

gravity-forms-wordpress-blocked-words.php

/* Remember to set your list of blocked words in WordPress > Settings > Discussion > Disallowed Comment Keys */

/* Gravity Forms Blocked Words */

add_filter( 'gform_validation', 'validate_disallowed_keys' );
function validate_disallowed_keys( $validation_result ) {

    $form = $validation_result['form'];
    $blacklist_keys = get_option( 'blacklist_keys' );

    foreach ( $form['fields'] as &$field ) {

        // Get the field value
        $field_value = rgpost( 'input_' . $field->id );

        // Check if the field value is disallowed
        if ( wp_check_comment_disallowed_list( '', $field_value, '', '', '', '', $blacklist_keys ) ) {

            // Mark the form as invalid
            $validation_result['is_valid'] = false;

            // Mark the field as invalid and add a custom validation message
            $field->failed_validation = true;
            $field->validation_message = 'We do not accept messages about link building services, content creation services or guest posts.';

        }

    }

    // Return the modified validation result
    return $validation_result;

}

Updated to use wp_check_comment_disallowed_list (); instead of wp_blacklist_check ();