If you're using Gmail's “Send As” feature to use a different SMTP server for outgoing mail, then yes, you must have a proper DMARC TXT record configured on the domain you send from (even if it's just `v=DMARC1; p=none`).

However, it is worth mentioning that Gmail is fully dropping the ability to use a non-Gmail SMTP server as of January: https://support.google.com/mail/answer/22370.