Gravity Forms opens with a setup wizard that asks for a license key. You do not have one, and you do not need one. Leave it empty and carry on.
What to do
On the license screen, enter nothing and continue. Gravity Forms works without it. The key sells you Gravity Forms’ own support and their update service, and neither is what you bought.
On the background updates screen, turn background updates off. That setting only controls updates coming directly from Gravity Forms using their license key, so with no key it does nothing except sit there failing. Gravity Forms shows a warning and asks you to confirm. Tick the box and continue.
Your updates arrive through the 99Plugs Update Manager instead. See How to enable auto updates.
The rest of the wizard is ordinary setup and none of it needs anything from us.
The prompt comes back
It will, from time to time, in the dashboard. It is a nag and not a warning. If it bothers you, hide it.
The add-ons
Gravity Forms add-ons install the same way and follow the same rule: no license key needed, updates through the Update Manager.
Why none of this breaks anything
A license key from a plugin developer was never permission to run the software. It is a subscription to their updates and their support. Your forms, entries, notifications and add-ons all work exactly as built. See Why are plugins and themes asking me for a license key?