I found this...

http://www.moddb.com/mods/zaero/downloads/zaero

Is it true that Zaero was a free download? Can't find any info about it?

people on other sites say the game eventually went free on the developer site, but the site went down at some point.

It was meant to be a commercial mod, but something went wrong. The story is the same as with Abyss of Pandemonium mod for Quake 1. You may torrent it freely without risking your soul getting to digital hell for it.

Here is some info for you (download links are dead):
https://www.quakewiki.net/archives/teamevolve/zaero_manual.html

You may also find "interesting" another unofficial mod called "Juggernaut: The New Story for Quake II"

Thank you, gonna take a look :-)