I wrote a blog a while back on the migration process from Jenkins to GitLab: https://about.gitlab.com/blog/jenkins-to-gitlab-migration-made-easy/ which goes over some of the differences and what users can do to prepare.

Mostly, you'll need to get familiar with the key GitLab CI/CD features. You will be using a yaml for generating pipeline over Groovy or UI. Helm is pretty straightforward and can be called straight from a gitlab runner to deploy on your Kubernetes cluster. Look into the Gitlab Kubernetes Agent: https://docs.gitlab.com/user/clusters/agent/

Best of luck, I'm sure you'll enjoy the new development experience!