Am using google appengine to develop and send mails, using the sandbox domain from mail gun approved test domain, was working. Now, am moving to production added the domain to mail gun from the dashboard , followed the doc from https://cloud.google.com/compute/docs/tutorials/sending-mail/using-mailgun, followed the documentation but the domain is still unverified, and also observed the doc is to send using an instance, and my instance is in dynamic scaling mode can't access my endpoint instance as it is dynamic, a bit confused on how to go about the domain verification of mail gun with google appengine or where to set TXT or CNAME record or even do DNS settings
Mailgun verification is independent of Google App Engine. You need to go to the company that you bought the domain from and add the TXT CNAME and DNS settings a la https://help.mailgun.com/hc/en-us/articles/202052074-How-do-I-verify-my-domain-
If you are not using a custom domain, I don't think google allows you to access / edit the DNS records probably because the .appspot domains are intended for development and testing.