You point your domain to Gmail. Then when they ban you, you point your domain instead to the new place, like Fastmail. So it's not really redirecting from Gma

You point your domain to Gmail. Then when they ban you, you point your domain instead to the new place, like Fastmail. So it's not really redirecting from Gmail, it's just changing a DNS record. On sending from a custom domain, your mail provider walks you through setting up a few additional DNS records (for DKIM, SPF) that allow you to send from your domain through their servers. They maintain the reputation of their IPs and so you get their good deliverability.