Is GliDOS HQ Ambience & Sound Effects free?

Yes. GliDOS HQ Ambience & Sound Effects is released under Open Source (BSD-2-Clause) and costs nothing. Neither it nor OpenLara has any paid content.

What do I need to run GliDOS HQ Ambience & Sound Effects?

OpenLara, which is itself free. GliDOS HQ Ambience & Sound Effects is an add-on and will not run standalone.

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

Roughly 75 MB, on top of the base OpenLara installation.

Yes. The source is published at github.com/XProger/OpenLara under Open Source (BSD-2-Clause).

GliDOS HQ Ambience & Sound Effects — Free Tomb Raider 1+2+3 Mod · PlayBound