Open Gravity in a browser. The license validation page appears.
Click on the button "Get licence". A new tab is opened and you are redirected to the "Purplen portal". Sign up or sign in.
Click on the button "Subscribe".
Choose the plan that fits your needs.
Fill the form that appears and validate it. For the "Free" plan, no credit card is required.
When it is done, you can close the tab and go gack to Gravity. Click on the button "Recheck".
When the validation is finished, you should see the following screen :