You can also design software A to produce both G1 and G2 side by side, deploy it, and then develop new software B against G2, submitting bug reports to project A when there’s a problem detected in G2.

If you’re doing the multirepo strategy it’s best imho to make the projects truly independent, as if they were developed by different companies. That way every project only needs to think about its own dependencies and consumers, and how to do migrations, without needing to have the big picture mapped out.