Obtaining the JARs manually is not difficult. However, I'm struggling to describe the process in the form of a Gradle build script. I'm still in the early stages of learning Gradle, and it's not obvious to me how to translate that GitHub URL into something that will cause Gradle to download all those asset files (in a portable fashion).
If asset JARs can be published on bintray, doing so would resolve my issue with jme3-testdata.
I'm also surprised to learn that jBullet isn't redistributable, as it appears to be included in the SDK distribution.