Currently there's no official out-of-the-box integration available in Gitlab that would allow you to post messages via Matrix protocol. There are some 3rd party options available, like Gitlab plugin for Maubot, or the Slack-compatible Webhooks Matrix appservice, but it might not be ideal solution in some cases for one reason or another.
So Internet has failed you and you ended up here? Well that's a bit awkward situation we have here, isn't it? I'd offer you some bisquits, but I ate them all recently.. So perhaps have a look around. Feel free to be nosy, if you'd like to. Read something if you feel like.. I'll be around, so ping me if you need anything.