At my work we use TeamCity for some things and Gitlab CI for others. Things that are good about TeamCity:
- Templates
Gitlab has something called templates but it's a very different thing. In Gitlab, a template is used to bootstrap a project, but that's it. In TeamCity a template is attached to a project such that if you change the template, changes are applied to all projects that inherit from the template. Each project can override any settings or build steps it got from the template, without losing the association to other settings. A project can have multiple templates attached to control orthogonal aspects of its behavior. From a template, you can see what projects inherit from it, and you can freely detach and attach to a different template. It makes managing a large number of projects with similar configs, that all evolve at somewhat different rates really easy.
- Build results
Teamcity has very good integration with xUnit and code coverage tools to quickly see test results and coverage as part of a build. Gitlab recently got better at this (it can now at least parse xUnit results), but you can still only see test results in the merge request view. TeamCity can also do things like track a metric over time and fail a build if it drops (i.e. PR builds should fail if code coverage drops more than X %). TeamCity also supports adding custom tabs to the build page so that you can attach reports generated by the build easily viewable in the UI (vs in Gitlab where you have to download the artifact and then open it to view)
- Overall view of runner status
It's very easy in TeamCity to see the build queue, and an estimate of when your build will run, and how long it's expected to take based on past builds.
-Dashboard
For me it's easier in TeamCity to see the overall status of deployments to a set of environments (i.e. what's on dev/stage/prod) that might span multiple source code repos. At a glance I can see what changes are pending for each environment, etc. In Gitlab things are too tied to a single repo or a single environment, and the pages tend to present either too much or too little information. Also, in TeamCity I can configure my own dashboard to see all of the stuff I care about and hide other things, all in one place.
- System wide configs
There are some settings that apply to the whole system (repository urls, etc). There's no easy way in Gitlab to have system wide settings, they have to be defined at the group or repository level. In TeamCity, you can configure things at any level, and then override at lower levels.
- Extensibility
TeamCity supports plugins. I know this can lead to the Jenkins problem of too many plugin versions, etc, but in TeamCity you tend to use far less plugins, and the plugin APIs have been super stable (I've written plugins against TeamCity 8 which is 4 major versions old and they work fine on the latest). It's really nice to be able to write a plugin that can perform common behavior and have it easily apply across projects and be nicely integrated into the UI.
To me, overall Gitlab CI seems useful for simple things, but overall it's 70% of the way to being something that could replace TeamCity.