Make sure to enable Bearer Token authentication on the Ruuvi Gateway device (and take note of the token).
You may also wish to enable scanning for all BLE beacons (not just RuuviTags); be sure to set the “Use Ruuvi Cloud or/and a custom server and configure more settings” option on the Cloud Options setup page, and when you get to the “Bluetooth Scanning” page, select “All (including third party beacons)”.
To add the Ruuvi Gateway integration to your Home Assistant instance, use this My button:
Ruuvi Gateway can be auto-discovered by Home Assistant. If an instance was found, it will be shown as Discovered. You can then set it up right away.
If it wasn’t discovered automatically, don’t worry! You can set up a manual integration entry: