Well, it isn't possible for anyone to register with your mobile number on their gmail account without any confirmation from you. You either receive a mail on your ID or a text on your mobile number, regarding the registration. So don't worry, but fo not forget to upgrade your privacy settings from time to time.
Though, if you received any confirmation code regarding the access and registration of your mobile number to a gmail account, without your knowledge, it is indeed, a matter of concern. Report it.