Paradox Interactive are celebrating the 20th anniversary of the Europa Universalis series so they're giving away Europa Universalis II and putting a bunch of other games on sale.
For the free game, simply head to GOG.com and find the big banner. It doesn't support Linux like Paradox's later games, since it's a proper classic from way back in 2001 but you can try your luck with the Wine compatibility layer if you really want it.
GOG have a big list of Paradox titles here that are currently on sale, which has some of the most popular mopdern Paradox strategy games going, along with some of their earlier games.
They also have them in bundles:
The GOG / Paradox sale runs until October 24, if you buy 3 or more titles you get an extra 10% off on top of the existing discounts.
Paradox and their Paradox Development Studio are also doing a big celebratory livestream on Twitch. Sounds like they have plenty of secret projects ongoing.