You can use your GOG.com account to login to GOG Galaxy 2.
The GOG.com integration in GOG Galaxy 2 is an official build-in integration made by GOG (part of CD Projekt Group). There are currently 2 official integrations in GOG Galaxy 2: GOG.com and Xbox Live.

All the other integrations such as Steam or Origin, are community integrations.
This means that the integrations are managed by plugins made by the community.

Note that logins used to connect the platforms to GOG Galaxy 2 using community integration are not shared with anyone.
The data is stored locally on your computer by using web browser session cookies.
Just as if you were logged in with Firefox or Chrome.