Hello,
With a standard VPC attachment you are bound to the VPC's public routing. The attachments point to a subnet so you can only point to one ENI at a time. This limits you to an HA solution, you cannot use auto-scaling.
We use VPN because in AWS peering is not transitive. The better way to pass traffic across multiple CP instances is VTI (Virtual Tunnel Interfaces). TGW comes with some sort of VPN GW and has ECMP, which will load balance traffic from any source VPC to CP GW. We also use VPNs because we have automation that allows us to build tunnels with the CME (see the Cloud Management Extension R80.10 and Higher Administration Guide). It will automatically take care of everything when a new auto-scaling instance is in the auto-scaling group. It will trigger a script to build site to site VPNs, advertise routes, and more.
In terms of resources, I attached a PDF made by Cloud Security Architect Eugene Tcheby that goes over the differences between TGW-ASG and TGW-HA. You can also check out this webinar which goes over what is required to deploy an auto-scaling group of CG GWs:
Best Regards,
Maya