To get started with building AOSP GSI, you'll need to get familiar with Git and Repo as well as How to build a GSI.
mkdir aosp; cd aosp
git clone https://github.com/ponces/treble_aosp -b android-15.0
bash treble_aosp/build.sh
These people have helped this project in some way or another, so they should be the ones who receive all the credit: