Can i use gmail as SMTP server for wsus ?
Sure - you'll need create a user account for it and use authenticated SMTP, but there's no reason it wouldn't work.
In WSUS, Under Options->Email Notifications, this screen has all the configuration options you should need to set up sending through gmail. For details, see the "Outgoing Mail Server" section on this page. Also, don't forget to tick the "authentication" box and provide your username/password.