We use GitLab for both internal and client project code, including their runners to run limiting on code (for example) so web based edits/commits can be checked with the same rules we may run locally. Love it.
We use GitLab wikis to replace Confluence (which had replaced Evernote because, well, I couldn't trust them any more). We sometimes use Gollum locally because sometimes a UI around editing pages is helpful, though I sure wish prose.io connected to GitLab. I'd also like to see GitLab switch to Kramdown for Markdown rendering (there's an issue open for that already) and actually use the Gollum gem to render the pages, so that we can use the various partials that Gollum supports when it's serving up a wiki.
Once time tracking lands in GitLab, our (admittedly) basic usage of Jira can be replaced, since we're already using guest/reporter accounts for customers to report all issues on any custom dev project we do for them. We'd like to see that on-boarding process (for our clients who we intend to use Issues) go a little more smoothly, and I'm working on a written spec for proposed improvements, and I'm comfortable enough with their code base to take a whack at writing it.
If it's not dev work (e.g. a general SharePoint support contract), we just use a new HelpScout mailbox with a custom email address for the client. That email address is just an alias that forwards to the primary HelpScout mailbox, where a workflow rule moves it to the client's specific HelpScout mailbox. Nice thing is that HelpScout's Slack trigger doesn't fire until after the email is moved, so notifications also go to the channel we use to discuss that customer's issues.
If Mattermost got bundled with their hosted offering, that'd be killer.
We're using their free hosted service, and so I'm reticent to ask for these sorts of changes by opening issues unless I want to at least attempt to contribute the code for it. I'm unsure how some of that would work out in their multi-tenant environment, but I still plan to dig into it.
GitLab is good stuff. I've been deeply impressed by their agility and responsiveness to their customers. If anyone is on the fence, there's no cost to check out early all of their features.