In my experience this mostly doesn't exist. For a public open source-only project, sure, you can use a variety of free simple CI/CD solutions. But as soon as you need anything more complicated than "run a public linter on my public code", you need to add plumbing that makes it complicated.
I am very interested in making the simplest possible CI method and deployment method, but I've never been able to make or use anything that wasn't complicated. The least-complicated solution I have ever seen is Drone.io, but even that's not all that simple.
At some point it is easier to install a gitlab omnibus than having to worry about the rest
I've been working on an MVP that is exactly this. By decoupling the CI (CD coming in soon) it is easier to run a tool like Prettier on every repo or a subset of repos in your GitHub organization. Check it out at githaxs.com.
The project seems great, I was a bit down after understanding that this was a gitlab-ci only venture (as I am currently searching for a simple and working CI/CD decoupled from the repo). But helping reduce the cognitive load needed to write good pipelines is always nice.