PX4 GPS to Arduino

PX4 Autopilot
2

Hi Jack,

  1. Exactly what kind of data can be output from a PX4?

Anything you want that the autopilot knows about.

  1. Does the information include GPS coordinates, including critical waypoints specified in Mission Planner or similar GCS?

Yes.

  1. If so, how can these waypoints feed to an Arduino in order to trigger pre-configured procedures?

Could you please explain a bit more detailed what you want to do? Do you want to send the entire set of waypoints in the current mission to the arduino? Or just a command when some particular wp is reached? You can also read up on mavlink, the protocol used for the communication between the autopilot and the GCS.
https://mavlink.io/en/