This one: https://docs.gradle.org/current/javadoc/org/gradle/api/attributes/plugin/GradlePluginApiVersion.html#GRADLE_PLUGIN_API_VERSION_ATTRIBUTE

Or do you mean how to set it for the only available feature variant?

On requesting it is set to the version of running Gradle

On publishing you can use it to require a minimum Gradle version, or to have multiple variants of the plugin for different Gradle versions by setting that attribute accordingly and changing the capability of the additional variants to be the same as the main variant, so that the only difference is that attribute.

Or maybe also the Java compatibility attribute if you also want different variants for different Java version executing Gradle

You can always check with the

outgoingVariants

task which variants are going to be published and whether they are like needed.

So to be more concrete, for the sole variant of a plugin:

configurations.apiElements {
    attributes {
        attribute(GRADLE_PLUGIN_API_VERSION_ATTRIBUTE, objects.named("8.5"))
    }
}
configurations.runtimeElements {
    attributes {
        attribute(GRADLE_PLUGIN_API_VERSION_ATTRIBUTE, objects.named("8.5"))
    }
}

For the multi-capability one, the one where the plugin supports multiple versions of Gradle, I would be able to do that using the “java features plugin” (I dont remember whats its called exactly, jus that it exists) right?

Or rather, it is not a separate plugin, is part of the

java

plugin

You just define feature variants and on the variants set the capability to the same as the one on the main variant and the Gradle version attribute to the respective value

or better put, how would I have figured out what configurations to put this on?

You could also use

configurations.matching { it.isCanBeConsumed } .configureEach { ... }

but imho that is too big a hammer, as it also sets it then on javadoc, sources, and so on and there is not really a reason to set the attribute on those