Most people use their Gateway ID to deal with their tax affairs online. When you first set up your account with the Government Gateway, your user ID would have been sent to you, either by post or by email. It would have been a long string of up to 12 numbers.
You could look through your email inbox, and search using the subject heading "Government Gateway user ID". If you can't find the email, you could telephone the appropriate HMRC helpline telephone number.
1. The main telephone number for HMRC is: 0300 200 3600
2. There are different numbers depending on which service you want, e.g. Self Assessment, PAYE or other areas of taxation. You can use this useful link HMRC contacts here to find the relevant helpline number.
3. Alternatively you can use this link for useful guidance to help you sign in https://www.gov.uk/log-in-register-hmrc-online-services/problems-signing-in
If you can't find your old ID, don't worry. It's a simple process to create a new tax account.