You have multiple options to checkin/checkout repositories. This page should give you an overview.
1. SSH-Checkin/Out: Please see https://docs.gitlab.com/ee/user/ssh.html
2. Checkout via HTTPS
***Important note**: You can not use your TU Graz online credentials.
* Create a "Personal Access Token": Please see https://docs.gitlab.com/ee/user/profile/personal_access_tokens.html for details.
* Create a "Deploy Token": Please see https://docs.gitlab.com/ee/user/project/deploy_tokens/ for details.
* Create a "Project access token": Please see https://docs.gitlab.com/ee/user/project/settings/project_access_tokens.html#create-a-project-access-token for details.