functions.php

functions.php_login_validate_field.php

<?php

add_filter( 'gform_field_validation_3', 'login_validate_field', 10, 4 );

function login_validate_field( $result, $value, $form, $field ) {
 
 global $user;

    if ( strpos( $field['cssClass'], 'email' ) !== false ) {

        $user = get_user_by_email( $value );

        if ( empty( $user->user_login ) ) {
            $result["is_valid"] = false;
            $result["message"] = "Sorry, we do not recognise that email address.";
        }
    }

    if ( strpos( $field['cssClass'], 'password' ) !== false ) {
        
        if ( !$user or !wp_check_password( $value, $user->data->user_pass, $user->ID ) ) {
            $result["is_valid"] = false;
            $result["message"] = "Incorrect Password.";
        }
    }

    return $result;
}