It's definitely true that the product focus on Gitter has always been more on public rooms. That's because the focus of our company has always been on the network of public communities, rather than enterprise features, which we feel are well represented in the market by other great products like Slack, Mattermost, etc.
(Background: I'm CTO/Cofounder at Gitter)
Forget slack - look at hipchat which has far simpler privacy controls. The ability to create an organization (NOT linked to GitHub), per channel permissions is all that's needed. Hipchat makes this dead simple.
Lots of different people will have different requirements - some people will need Active Directory even, but essentially the only two things needed for enterprise messaging is permissions and search.
EDIT: Previously said open source, open core is the correct term.
Gitlab is incredible... but I really cant host right now. Same for mattermost.
I do believe that the ability to run a fairly large hosted system at scale and with incredible performance is what the gitter guys bring to the table.
(disclaimer: work at Discord & fishing for feedback about how we can make this easier)
I linked to a ticket - if you follow that trail, there are many more.
