In my settings.gradle.kts, I can do this to check if a property (in ~/gradle.properties) has been set, right?

val artifactoryUsername : String? by settings
if(artifactoryUsername == null) {
   ...

How can I do the same in groovy dsl? (Sorry, not all builds converted to kotlin, yet)

I haven't used the Groovy DSL for a while, but I believe you can use

if (project.hasProperty('artifactoryUsername')) {
   // do something
}

Thanks for the answer! Unfortunately, that works only in a build.gradle, but not in settings.gradle, because there is no project.

Yup, that's it.