Is Cessna 172P Skyhawk Detailed Edition free?

Yes. Cessna 172P Skyhawk Detailed Edition is released under GPL-2.0-or-later and costs nothing. Neither it nor FlightGear has any paid content.

What do I need to run Cessna 172P Skyhawk Detailed Edition?

FlightGear, which is itself free. Cessna 172P Skyhawk Detailed Edition is an add-on and will not run standalone.

The PlayBound Launcher installs it in one click from FlightGear's mods list. You can also install it by hand — full steps are on this page.

Roughly 110 MB, on top of the base FlightGear installation.

Yes. The source is published at github.com/c172p-team/c172p-detailed under GPL-2.0-or-later.

Cessna 172P Skyhawk Detailed Edition — Free FlightGear Mod · PlayBound