It would be good to user the new G Suite ( GMail Addons ) for showing data related to the user, who's email that you are viewing. For Example, a sales user, would when opening an email from a user see address, phone number in the Addon. They could also see that they have a task to set up a meeting.
There is form dev level info here https://developers.google.com/gmail/add-ons/
I have done a POC, and I can get data from the api using appscript, via an api_key. When I have time I will work on it a bit more.
I may add a forum post if anyone would like to follow my progress on this.