I’ve set up the new Cloudflare email routing with one of my domains and am currently forwarding it to one of my gmail accounts. However, I can’t send from my gmail account as an alias with this domain because it is asking for SMTP information.
Routing is only a forwarder for inbound mail. It’s not an outbound relay. You would need to figure out a way to get Gmail to do a “Send As” through their system, or find some other relay host.