This is just a note for anyone running into the same issues that I was running into.
There are point in documentation that suggest running /platform -version, or similar things. This just adds confusion when dealing with GitLab Mattermost. Here’s what worked for me:
sudo -u mattermost ./mattermost -config=/var/opt/gitlab/mattermost/config.json
This was the only way to get it to work. In other words, “sudo -u mattermost ./mattermost” takes the place of “platform” anywhere you see it in documentation, and you have to append -config at the end.
Hopefully this will help clarify things for other users.