Not exactly newsletter is a source of this, but perhaps it's worth to be mentioned here:
https://www.gog.com/redeem/CHICKENPOLICE55OFF
Chicken Police -55%

Thanks a lot. (Morrowind)

Another code for:

Per Aspera (-55%)
Star Renegades (-65%)

526VspaceKYLMXgamesPLYRLgATN4

Remove lower case letters to use, expires April 21, 2022, at 1 PM UTC.
-----------------------
And here's another for Winter's Ember (-20%), expires April 22, 2022, at 1 PM UTC.

5**U7FBC96EF1??75F

?? = the reverse of ninety six

** = Burger King

Postal 2

%%BE6-1E%$%-E3C77-58$8E - % = 4, $ = 9

Gift certificate expires on May 22, 2022 (11:59 PM GMT).

Winter Ember (20% launch discount)

code: 5BKU2ARTHUR2968CC3E143AB
^ to play as ARTHUR, simply revoke his name to redeem your discount code

Offer expires on April 22nd, 2022, at 1 PM UTC.

Thronebreaker -75% $4.99

TBEQ0F39C2752BD55A

Winter Ember
5BKU1E2AA55F953BBF

Thronebreaker
TBEQ1E304E34F054FA

5BKUCOAL04EAA247B04710 for winter ember but you have remove COAL also expires in 15 hours or so, so you need to hurry

Looks like a new low for Thronebreaker. Maybe its time to pick it up.

Thronebreaker: The Witcher Tales

T!EQ17F9!9A927!2DC - ! = B

The offer ends on April 24th, 2022, at 1 PM UTC.

Ugh! Went back to the Morrowind email / newsletter this AM and saw I missed it. Expired. Oh well... another time I guess.

Discount for Thronebreaker:

TBEQ5A90BECDF9D86*

* is the missing letter in ABCEFGHIJKLMNOP

Expires April 24th 2022 at 1 PM UTC.

Here's yet another discount code for Thronebreaker:

TBEQ421A7B1B17E373

For the newcomers
https://www.gog.com/en/welcome-offer

Another discount code for Thronebreaker: The Witcher Tales (-75%):

TBEQzzz7AB7AC94976679 (remove zzz)

The code expires on April 24, 2022, at 1 PM UTC.