You will need GMC registration/licensing before the advertised post start date, 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 the required date.
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.