We're trying to achieve two things with open sourcing Gitter.
Firstly, we see Gitter as more of a community than a product. That we have nearly 1,000,000 developers coming together on the platform to talk about code and open source software where public rooms far, far outweigh private rooms, is testament to that. So we're not trying to compete with MatterMost.
What we're trying to facilitate is to allow the community and users of GitLab to contribute to the experience of the community. We've always had a very small team at Gitter and the surface area of the product is massive. This way the community can help us make it a better place by improving the product.
We also have some communities who are interested in running their own public instance entirely. Sure they could use other products, but Gitter is built community and public first and suits a lot of different needs that other products don't necessarily facilitate out of the box.