If you want to delete the file, of course it won't be deleted using those functions, you need to use unlink():
unlink('path/to/the/file.png');
Obligatory Note:
Please, don't use mysql_* functions in new code. They are no longer maintained and are officially deprecated. See the red box? Learn about prepared statements instead, and use PDO or MySQLi - this article will help you decide which. If you choose PDO, here is a good tutorial.
First, (and I hope) you need to have saved the filename of that file that you want to be deleted.
Then before deleting the row, you could get that filename first. Then just add/concatenate it in the path that the unlink() will be using:
<?php
// connection
$con = new mysqli('localhost', 'username', 'password', 'database_name');
if(isset($_GET['module'], $_GET['act'], $_GET['file_id'])) {
    $module = $_GET['module'];
    $act = $_GET['act'];
    $Gfile_id = $_GET['file_id'];
    if($module == 'galleryPhoto' && $act == 'delete') {
        // get the file path first that is saved
        $sql = 'SELECT file_path FROM gallery_photo WHERE file_id = ?';
        $select = $con->prepare($sql);
        $select->bind_param('s', $Gfile_id);
        $select->execute();
        $select->bind_result($file_path);
        $select->fetch(); // fetch single row
        // delete the file
        unlink('path/to/images/' . $file_path);
        // then delete that row
        $sql = 'DELETE FROM gallery_photo WHERE file_id = ?';
        $delete = $con->prepare($sql);
        $delete->bind_param('s', $Gfile_id);
        $delete->execute();
        header('Location: ../?v=gallery_foto&info=Success');
    }
}
?>