So what collaborative features unique to Gmail does the organisation you work for currently actively uses?
PS: please do not say "Hangouts", as Hangouts are only loosely integrated with Gmail. These are two separate systems and it does seem they are still unsure if these two should be merged together.
Why do they have to be "unique to Gmail"? The parent's argument was that email is the same thing for 30+ years.
Several mail programs have added integrated calendars and notifications, email snoozing, labels, classification into buckets, context, and so on -- including basics like search.
And of course there are the apps and plugins -- e.g. Asana within Gmail and the like.
PS: please do not say "Hangouts", as Hangouts are only loosely integrated with Gmail. These are two separate systems and it does seem they are still unsure if these two should be merged together.