Entries like in Gradle version catalogs .toml file

[overpas]

Is it possible to work with toml code like this?

[libraries]
four-koma = { group = "cc.ekblad", name = "4koma", version.ref = "four-koma" }

Or should { group = "cc.ekblad", name = "4koma", version.ref = "four-koma" } be treated as a TomlValue.String?

[valderman]

Yes, your first example will turn into the following:

TomlValue.Map(
    "libraries" to TomlValue.Map(
        "four-koma" to TomlValue.Map(
            "group" to TomlValue.String("cc.ekblad"),
            ... 
        )
    )
)

Note that version.ref = "four-koma" maps to "version" to TomlValue.Map("ref" to TomlValue.String("four-koma")). If you want to use version.ref as a literal key, you need to quote it.

In general, 4koma should follow the TOML 1.0 spec to the letter. If it deviates from it, that's a bug and should be reported as such.