So if I'm not using Gradle I need to add the library explicitly?

You can't not use Gradle if you want to use kotlinx.serialization.

Ugh.

That's how it works. A Gradle plugin generates the serialization code so you don't have to hand write it. It seems like things are unclear to you.

I wasn't aware it was using a Gradle plugin to generate the serialization code.

Well, it's documented: https://github.com/Kotlin/kotlinx.serialization#gradle

The readme says there's an example Maven project too.

IDEA handling the build, but to add compiler plugin and runtime library to build classpath, you'd better use Gradle