Here is the list of services that we use https://about.gitlab.com/handbook/business-ops/tech-stack/
This includes many proprietary ones, we generally choose the product that will work best for us, considering the benefits of open source, but not excluding proprietary software.
Mattermost is not part of the single application that GitLab is. There is a good integration between with GitLab and our Omnibus installer allows you to easily install it. But it is a separate application from a separate company.
They use a ton of services.
Likely you don't want your backup to be one of your systems and another part of the company probably uses Zoom already so it is probably easy to fail over to that.