For the record: You can also copy paste your gitlab-ci.yml into the linter-form provided by gitlab:
Depending on which IDE you are using you might be able to find plugins that check for validity. For example in VS Code you can use a plugin called gitlab-vscode-extension which can validate your .gitlab-ci.yml file.
In case you want to programatically validate your .gitlab-ci.yml, gitlab provides an API which allows you to POST your yml to /ci/lint, e.g.:
curl --header "Content-Type: application/json" https://gitlab.example.com/api/v4/ci/lint --data '{"content": "{ \"image\": \"ruby:2.6\", \"services\": [\"postgres\"], \"before_script\": [\"bundle install\", \"bundle exec rake db:create\"], \"variables\": {\"DB_NAME\": \"postgres\"}, \"types\": [\"test\", \"deploy\", \"notify\"], \"rspec\": { \"script\": \"rake spec\", \"tags\": [\"ruby\", \"postgres\"], \"only\": [\"branches\"]}}"}'