Re:GitLab acquires Gitter, will open-source the code
by sytse on Wednesday March 15, 2017 @01:27PM
(#13877945)
Mattermost is developed by another team https://about.mattermost.com/company/ that is separate from the GitLab/Gitter team. So I think both will continue to grow without affecting each other.
They surely have the best intentions. Hopefully one product doesn't take priority over the other and/or they can prevent cannibalization. There's definite overlap.