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Hard to say without seeing your build script or an MCVE. Usually you have a list of repositories and Gradle checks one after the other for the dependencies. If it gets a server error for one repo like here, it might be the case that it is not looking further, but errors out. Usually plugins should not silently add repositories to the list though, that's very bad practice for several reasons.

repositories {
    mavenCentral()
    gradlePluginPortal()
    // Spock snapshots are available from the Sonatype OSS snapshot repository
    maven { url "<https://oss.sonatype.org/content/repositories/snapshots/>" }
    maven{
        url "<https://packages.confluent.io/maven/>"
    }
    // needed for dataflow
    jcenter()
    ivy {
        url '<https://dl.bintray.com/content/johnrengelman/gradle-plugins>'
    }
}

I would have thought that Gradle would try the repos in order of there definition... is this wrong?

No, that's correct

And I know

org.apache.spark:spark-core_2.13:3.1.1

is in Maven Central

Well, not sure, but you should probably remove that bintray repo anyway, iirc all bintray repos were shut down which is probably also the cause you get a 502 error

Doh!