From the e-mail: "(1) We’re adding a new functionality - GOG Direct to Account that will enable partnered platforms to activate, directly on your GOG.COM account, content purchased via these selected third party platforms"

Where do I find this to activate games I've already paid for for use with my GOG account?

Some third party stores sold GOG keys and you had to redeem them manually.
Now those stores could redeem GOG games automatically for you.

Oh. Is that all.

Thank you very much for explaining it to me.

It probably means something like buying a game on humble bundle which then activates directly into your GOG account (without providing a key - which would expose the keys being sold for example on G2A)

The other things are called GoG Reclaim (a few titles that if you ownt he DVD/CD version you can use the key here.)
https://www.gog.com/reclaim

And then GoG Connect which lets you link your Steam acount and activate some games that you already bought there (this requires the publisher/developer to allow it):
https://www.gog.com/connect

And yeah, this new thing sounds like allowing sites like Humble to connect and auto add the game to your account without manually doing it yourself via cd-key.