Third party apps may be reading our Gmail. If this message does not bother you then,ignore this message.

Third-party developers often reading a Gmail message content, once relevant authorization is available to them.

To increase your privacy, use Security Checkup

To get access to this tool, you will need to sign in to your Google Account (Myaccount.google.com) and manage specific settings.
(There is one feature to be noted above the others as it shows the number of applications having access to the data)
In order to proceed, click the squares at the right upper corner, and choose Account in the drop-down list. Continue with finding Security Checkup tab. This option enables viewing the number of devices used for signing in your Account. It also informs whether any security issues have occurred over recent 28 days.

https://www.hackread.com/third-party-apps-may-read-your-email-learn-how-to-protect/