Can we use an existing gitlab.yml file as it as in github for CI? Will the CI pipeline work if I migrate my entire repo from Gitlab to GitHub? Or I should write the .yml file anew? I am completely new to this.
GitLab CI/CD is not compatible with GitHub Actions. You will need to write a new workflow. The following are some resources to get you started.
GitHub Actions Starter Workflows : This is a good place to start to get a base working example for the workflow you need.
GitHub Actions Official Documentation
GitHub Actions on the Marketplace : There are lots of useful 3rd party actions that you can freely use in your workflows.