Try *$removeparam=source and if that behaves as expected, add ,domain=gog.com at the end, though that seems unnecessarily specific. Also bear in mind that filter is supposed to modify the network request before it goes out. So go look at the network log in the developer tools to double-check if it's working instead of relying on the address bar.

There are also multiple browser add-ons that will strip the relevant bits from the links themselves, like ClearURLs.

Thank you Dev0_NZ & Ice_Mage!

As I dont play bookmarklets nor the developer tools,
please give me some time to get familiar with them to run your ideas

About ClearURLs, I used it time ago & something didnt like me,
if I remember correctly, it did not remove some trackers for some websites...
maybe I took notes... I need to find them :)

Basically,
1. Go to the aforementioned sales page with the links with the first-party tracking parameters.
2. Press F12 to bring up the developer tools, then go to the Network section. Over there, enter source=news in the Filter bar.
3. Click one of the links and see if it shows up in the network log.

Thank you
Well, it seems it gets redirected to the untracked link! :)

From today's news:
gog.com/en/news/release_4_great_classics_from_sneg_and_silver_box_classics_containing_4_more_all_with_15_sprin g_sale_discounts
The DragonStrike link
(Screenshots attached)

Then I got [it wrong] &/or [lost in the conversations]
from this gorhill posts (The developer):

github.com/uBlockOrigin/uBlock-issues/issues/46#issuecomment-391303700
github.com/gorhill/uBlock/issues/2279

And digging the documentation:
github.com/gorhill/uBlock/wiki/Static-filter-syntax#removeparam

"removeparam
New in 1.32.0.
...
removeparam is a modifier option (like csp) in that it does not cause a network request to be blocked but rather becomes modified before being emitted"

Ice_Mage: You not only told the way it works, also gave a way to prove it!
Thumbs up & Thanks for sharing!! :)