Any doctor wishing to practise medicine in the UK must be registered by the General Medical Council (GMC). In addition, the GMC must have provided them with an up-to-date licence to practise medicine.
You will need GMC registration/licensing before the intended post start date, ie August 2021, however you do not need to have gained this at the time you make your application.
If you do not already have full GMC registration when you make your application, you will need to indicate why you think you will be able to gain both GMC registration and licensing by August 2021.
On your application: within the 'Professional registration' section of the eligibility page, either give details of your GMC registration/licensing; or provide details of your eligibility for these if you do not yet possess them.
At interview: you will not need to supply any information regarding GMC status at interviews.