CNAME records usually come into play when you’re configuring Gainsight to send email on your behalf.

To control spam, most email services have checks they perform on arriving emails, to ensure the message really came from the person who claims to have sent it. If the email doesn’t pass a check, it could get marked as spam, or discarded outright. Because Gainsight is going to send mail that’s “From” you or your company or your CSM team, you use CNAME records to create a trustworthy signal that the emails sent by Gainsight’s tools really are legitimate, and should be treated as if they are sent from you directly.

The CNAME records create that trust, so your messages are more likely to be delivered to, and be opened by, and be valuable to your customers.

More here:

https://support.gainsight.com/Gainsight_NXT/01Onboarding_and_Implementation/Onboarding_for_Gainsight_NXT_in_Salesforce/Login_and_Permissions/Email_Service_Configuration