If you are not prepared to use Ruby/dpl you can deploy to Heroku as follows:

Look up your Heroku API key (Account settings -> API Key on the Heroku web console) and make it available as a Gitlab secret variable e.g. HEROKU_API_KEY (Please note the values is not the same as what heroku auth:token returns...)

Then add two script lines in your .gitlab-ci.yml config file at the relevant job:

git remote add heroku https://heroku:$HEROKU_API_KEY@git.heroku.com/.git

git push -f heroku HEAD:master
You can see detailed explanation at http://blog.thecodewhisperer.com/permalink/deploying-jekyll-to-heroku-using-gitlab-ci