You could, whether you should is a matter for you. For a personal site or one with few users you can get away with it. But Gmail wasn't designed with transactional email in mind, Gmail is not meant to be used in this way.
Hitting recipient's spam folders may be the least of your worries - Gmail may simply refuse to accept or deliver emails.
Hitting recipient's spam folders may be the least of your worries - Gmail may simply refuse to accept or deliver emails.