Under Settings > General in the Panther Console, I see a "Gateway Public IP" at the bottom. What is this IP address and what is it used for?
This is the IP address of the public-facing gateway that Panther uses if it needs to communicate with external services. For example, if your detection generates an alert, Panther will send this alert to one or more of your alert destinations. When your alert destination receives a message from Panther, it will see it show up as that Gateway Public IP address.
This is also true for detections: If one of your detections needs to make an external API call, that external service will see the request show up from the Gateway Public IP address. (Note: this is only true for live detections, not for test cases).