@jjohannes, thanks for offering to have a look and fill in what I'm missing. Here's what I've come up with. You can post suggestions for edits, or you can copy the whole thing and post a comment with your full, revised version.

Gradle Module Metadata

The Gradle team has contributed a metadata file for Guava. If you use Gradle 6 or higher, you will see better handling of two kinds of dependency conflicts:

Selecting the appropriate flavor

When Gradle automatically selects the newest version of Guava in your dependency graph, it will now also select the appropriate flavor (-android or -jre) based on whether you project targets Android or not. For example, if you depend on 32.1.0-android and 30.0-jre, Gradle will select 32.1.0-jre. This is the version most likely to be compatible with all your dependencies.

If you need to override Gradle's choice, you can TODO: something like the following? I thought I'd convinced myself that I understood which part(s) of it were necessary, but now I'm confused again, sorry:

Reporting dependencies that overlap with Guava

If your dependency graph contains the very old google-collections or the hacky listenablefuture, Gradle will now report that those libraries contain duplicates of Guava classes. When this happens, you'll need to tell Gradle to select Guava:

configurations.all {
    resolutionStrategy.capabilitiesResolution.withCapability("com.google.collections:google-collections") {
        select("com.google.guava:guava:0")
    }
    // and/or
    resolutionStrategy.capabilitiesResolution.withCapability("com.google.guava:listenablefuture") {
        select("com.google.guava:guava:0")
    }
}

(I could also mention that we'll now omit j2objc-annotations from the runtime classpath, but I think we may be better off focusing on the bigger changes, since there's plenty to discuss. Let me know if you disagree.)