macOS wants to access your Google Account

You will see this message on your Google account if you want Mac Mail to retrieve you emails from Google. You may also choose to sync your Google calendar and contacts with the Mac. This is not unusual and most email providers now have a security setting that you need to set if you want another app, such as Mac Mail to retrieve your emails.

Otherwise you do not need to add the Gmail account to Mac Mail And can just use the web to access your emails through your Google account.

As for safety, Apple is not interested in your emails at all, so you do not have to worry about your Mac compromising your privacy from those emails. Google now provides this message to you because they were caught giving your emails to third party's and not telling you. In addition up until 2017 Google was reading your emails to target personalized ads to you.

https://mashable.com/article/google-reading-your-emails-response