To generate your Age Gate go to this link.
Once there you'll be able to customize your Age Gate as you please including the text that you want to display on it.
Once satisfied click "Preview & Generate Code" and something similar to below should display:
Copy the code that appears and next we'll add it to the editor.
Select App Integrations
And then select Third-Party Integrations
From here click to add an integration and find Age Gate in the list:
You should now see a field where you can enter your code from earlier:
Add and click save. Click the toggle switch to activate your integration.
Now publish and your Age Gate should appear for your visitors on their first visit.