Hey, it is actually connected with GOG Galaxy. Your username will appear if you launch the game via GOG Galaxy while being logged into your account. However, once you log out and launch the game offline, your username should disappear. Is it possible that you still have some GOG Galaxy remnants in your system files? You can also try to uninstall the game and install it again BUT using new offline installer instead the one that you already have.

And of course it is not a DRM, no worries ;)