has anyone ever managed to include a git source repo in a gradle project?

I have. But maybe better asked in #gradle?

I just managed to get it working

Sharing your solution would be a way to give back to the community that you request help from

good point. In settings.gradle

sourceControl {
    gitRepository(uri("<ssh://git@github.com/MyOrg/mylib.git>")) {
        producesModule("io.myorg:mylib")
    }
}

and importantly, in build.gradle.kts:

implementation("io.myorg:myapp:1.0") {
                    version {
                        branch = "main"
                    }
                }