Good Old Games will be enabling two-factor authentication on customer accounts using GoG.com starting on Monday, 24th October 2016.
In an email sent out to existing customers, the digital store front said
On October 24th, we will start enabling Two-step login on all current accounts, with a goal to increase your account’s security. Once enabled, the first time you log in from a new device or network, we will send you an email with a unique code, which must be entered in order to proceed. If you’re using GOG mostly from home or work, you should only be required to use the Two-step code very rarely.
While it is possible to opt-out of the security change and not use two-factor authentication, GoG recommends everyone use it for the protection of their accounts. If you are determined not to use extra security measures on your account, they provide information on how you can opt-out as well.
Our Two-step login process is very user-friendly, so we suggest that you give it a try, but if you do not wish to use it for some reason, click this link (before October 24th) or disable it in your Account Settings (after October 24th). However, we strongly discourage this, so please be aware that you would be doing this at your own risk.
For more information on the changes and what it means, check out the Official FAQ.