Great intro https://www.gitops.tech/
For me the difference between Push-based (also CI-Ops, like traditional CI/CD pipelines) and Pull-based (operator takes over) Deployments was the key here.
https://www.dev-insider.de/gitops-tools-in-der-uebersicht-a-943715/
https://cloudogu.com/de/blog/gitops-tools
https://argoproj.github.io/cd + https://argo-cd.readthedocs.io/en/stable/
Watch out - there's old https://github.com/fluxcd/flux and new https://github.com/fluxcd/flux2
https://werf.io/ (by https://flant.com/)