You’ll have to prove your identity to register for Government Gateway if you have not used it before.
You’ll need your National Insurance number or postcode and 2 of the following:
a valid UK passport
a UK photocard driving licence issued by the DVLA (or DVA in Northern Ireland)
a payslip from the last 3 months or a P60 from your employer for the last tax year
details of a tax credit claim if you made one
details from a Self Assessment tax return if you made one
information held on your credit record if you have one (such as loans, credit cards or mortgages)