Is LittleNavMap integration free?

Yes. LittleNavMap integration is released under GPL / FlightGear add-on and costs nothing. Neither it nor FlightGear has any paid content.

What do I need to run LittleNavMap integration?

FlightGear, which is itself free. LittleNavMap integration 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 6 MB, on top of the base FlightGear installation.

Yes. The source is published at github.com/slawekmikula/flightgear-addon-littlenavmap under GPL / FlightGear add-on.

LittleNavMap integration — Free FlightGear Mod · PlayBound