Is General Dynamics F-16 Fighting Falcon free?

Yes. General Dynamics F-16 Fighting Falcon 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 General Dynamics F-16 Fighting Falcon?

FlightGear, which is itself free. General Dynamics F-16 Fighting Falcon 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 95 MB, on top of the base FlightGear installation.

Yes. The source is published at github.com/NikolaiVChr/f16 under GPL-2.0-or-later.

General Dynamics F-16 Fighting Falcon — Free FlightGear Mod · PlayBound