I have read a couple of articles on mixing in GB. They talk about sending the pre mix song/track out of GB and then bringing it in again to start mixing. Like that changes it somehow. What is the story with that?

Some people like to render (bounce down) any Virtual Instrument tracks as audio files before they start mixing—a lot of Pro Tools users used to do that as a matter of course; I assume that's what's meant by "sending the pre mix song/track out of GB and then bringing it in again". While doing that does reduce the overall CPU hit while mixing [especially when there are a lot of Virtual Instrument tracks utilizing heavy-CPU instruments], and makes it easier to share the individual tracks if you wanted to send them to someone else to mix in a bigger [different] DAW, nowadays I don't think there's any significant advantage to doing it [I don't do it in Logic], especially to mix in GB, unless your arrangement is very large and you're running into CPU issues.

understood