My understanding was that the two domains are equivalent. The following sites seem to confirm my understanding. Are you sure it isn't you?
https://support.google.com/mail/thread/125577450/gmail-and-g...
https://www.quora.com/What-is-the-difference-between-gmail.c...
https://www.gmass.co/blog/domains-gmail-com-googlemail-com-a...