Riot has everything Slack does, but it's more competent and free. And obviously the main selling point is compatibility with all kinds of other chats.

>Riot has everything Slack does

Gitlab integration?

Yes, of course. But unlike Slack you are not limited to "official" "integrations", you can add arbitrary widgets and can write your own.

At my workplace we use Rocket.chat and it does have GitLab integration, so there's another possible alternative.

I'm running the reference Matrix server (Synapse) on my home computer, and I'm using the Riot.im client. I mainly use it to communicate with a group of three friends. They are using Android, and I'm using an iPhone.

It seems to work about as well as any other messaging app, which is impressive if you ask me, considering it's completely open and self-hosted.

It's better than Slack in terms of usability, performance and UI (subjective), but lacks some integrations that Slack has. Search engine is quite poor (not that Slack is any better, it's shit too).