In order to trigger a Gitlab pipeline automatically, a so-called trigger must be created. You can create as many triggers as you want in the project - settings.
These triggers look like this. Of course, the (placeholders) must be replaced.
$ curl -X POST -F token=(TOKEN) -F ref=(BRANCH) http://(ip):(port)/api/v4/projects/(project-id)/trigger/pipeline
This curl call can be entered into the Synology task scheduler, Done!