Postman only delivering as OAuth'd account

OAuth with who, Gmail? Are you using SMTP or the API? I need more details.

Sorry

Gmail/google apps via the API

OAuth rules do not allow Email Spoofing. If you check the transcript, you can see Postman sends the From Address you want to use. However Google replaces the From Address you specify with your own. The name is left alone.

If you can prove you own the email address, Google will allow you to use it as the From Address with their Send Mail As feature. Also see the Treat As An Alias setting.

Both the gravity forms settings and Postman’s Envelope/Message From settings just get ignored

Envelope From is always ignored by the API. In v1.7, this field disappears when you use the API.

ref: https://www.ads-software.com/tags/send-mail-as

thanks! makes sense now.