I agree that Gradle can be a pain to package software for Debian.
According to the documentation of the antonov protobuf gradle plugin
[1], the name of the plugin is just "protobuf".
Ahhh... RTFM. Ha, ha! If I had been less tired and frustrated I should have thought of that. Thanks!
In the meantime, I wrote a simple autotools build. It's only two source files so it was fairly trivial. The fact that Gradle needed hundreds of lines to essentially build and test two C++ source files really had me shaking my head...
But I'll try the upstream build again soon with the updated plugin name, it's definitely cleaner to not have to reimplement an upstream build system!
-Olek