Is The Great War 1914 Scenario free?

Yes. The Great War 1914 Scenario is released under Open Source (GPL-3.0) and costs nothing. Neither it nor TripleA has any paid content.

What do I need to run The Great War 1914 Scenario?

TripleA, which is itself free. The Great War 1914 Scenario is an add-on and will not run standalone.

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

Roughly 30 MB, on top of the base TripleA installation.

Yes. The source is published at github.com/triplea-game/triplea under Open Source (GPL-3.0).

The Great War 1914 Scenario — Free TripleA Mod · PlayBound