I will try to answer your questions.
As you said, Gradle is a build tool, but the line you have mentioned in your first question is referring to
Gradle Java plugin version
. Gradle has plugin for building Java projects. These plugins extend upon the general purpose build tool Gradle is, and may provide additional features. You can read more about the Gradle plugins here.The reason your build fails when you change the plugin version is simple, the plugin version 0.8 is not there. There is a relationship between Gradle version and the plugin version and a developer has to maintain that. You can only use a plugin version which has been released and is compatible with the Gradle version. I hope the answer to first question answers this anyways.