Game I purchased require 'serial key', where I can get one?

Easiest way:

1. log into your GOG account

2. go to 'ACCOUNT' section

3. find your game

4. click on it

5. click on button 'MORE'

6. in drop down list select 'SERIAL KEYS'

Hard way:

1. install 'games-util/lgogdownloader': emerge games-util/lgogdownloader"

2. go to safe directory

3. run: lgogdownloader --download --exclude all --save-serials

4. enter login information

You will get all Your serial keys at once.