So the idea is you be able to easily upgrade or even multiboot different android or linux images if you wish, without having to recompile for every device.
I don't know how much of a success it has been, and I don't think it has been used to multiboot a phone or to boot a more standard Linux.
https://android-developers.googleblog.com/2017/05/here-comes...
I thought mobian was distributing a GSI to do non-Android on the Android base, but I can't seem to find it if they do that.
Also, bear in mind that this almost intentionally doesn't solve some support problems - any bugs/vulnerabilities in the base layer won't get fixed by GSIs (which is why DivestOS says it explicitly doesn't ship a GSI).