Is San Francisco Bay Area KSFO HD Scenery free?

Yes. San Francisco Bay Area KSFO HD Scenery 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 San Francisco Bay Area KSFO HD Scenery?

FlightGear, which is itself free. San Francisco Bay Area KSFO HD Scenery 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 150 MB, on top of the base FlightGear installation.

Yes. The source is published at github.com/FlightGear-Community/KSFO-scenery under GPL-2.0-or-later.

San Francisco Bay Area KSFO HD Scenery — Free FlightGear Mod · PlayBound