GTA 6 will probably be software as a service to some extend, namely the code will change all the time at somewhat high frequency for any port. If I am not too mistaken a big chunk of the GTA community does play online.

"AI porting" able to follow? (If that AI thing is actually true)

Unless GTA 6 is not hardwired to AMDGPU hardware at release, if they have a cross-platform engine, for PC, it is to click the right export button... and do the QA and debug to please the various 3D drivers out there.