Where is my Government Gateway?
Government Gateway is a portal through which you can access HMRC services. If you have one set up you will access it through this link: https://www.access.service.gov.uk/login/signin/creds.
It is however possible that you have not set one up in which case you will need to do so through these pages: https://www.gov.uk/log-in-register-hmrc-online-services/register
Each Government Gateway is attached to a legal entity. As an individual you may have one for your personal tax, if you run a Limited Company it can have its own one.