Not sure what you mean by "smallest". With GitLab you create docker images. That means you can easily run them locally and share them. In the example you gave the GitHub actions one looks like more lines of YAML but only works because they made an "action". If they had provided a Docker image then GitLab would be just as easy (and trivial to test locally).
mise already is trivial to test locally though, and I'm not sure I agree that maintaining a CI config + Docker image is just as easy as maintaining a CI config.