http://rafalgicgier.com/gf-front-end-login-register/

gf_val.php

<?php
// the _3 prefix has to match the id of the form you have created
add_filter( "gform_field_validation_3", "login_validate_field", 10, 4 );


function login_validate_field($result, $value, $form, $field) {

	// make sure this variable is global
	// this function is fired via recurrence for each field, s
	global $user;

	// validate username
	if ( $field['cssClass'] === 'username' ) {
		$user = get_user_by( 'login', $value );

		if ( empty( $user->user_login ) ) {
			$result["is_valid"] = false;
			$result["message"] = "Invalid username provided.";
		}
	}

	// validate pass
	if ( $field['cssClass'] === 'password' ) {
		if ( !$user or !wp_check_password( $value, $user->data->user_pass, $user->ID ) ) {
			$result["is_valid"] = false;
			$result["message"] = "Invalid password provided.";
		}
	}

	return $result;
}