Alright, so I guess the problem is that you are just grabbing the plugin’s jar file, but that jar file does not have references it’s required dependencies. There are a few other ways to get the plugin correctly on the classpath of the consumer project.
You can make a “fat jar” of the plugin which includes all dependencies. This is a rather ugly solution though as it can’t be used in dependency resolution rules.
If this is a plugin you are only using for that particular project, and you have the source code for the plugin is available with the project (e.g. in the same repository), you can also include it as a composite project. To do this, you will put a reference to the plugin in your settings.gradle file for the consumer project like this: includeBuild("../path/to/plugin").
Another solution is to publish the the plugin as a Maven distribution, and then depend the Maven coordinates in the consumer project. You could possibly just publish it to a local maven repository (e.g. the “.m2” folder in your home) to keep it simple.
Lastly, you can also just supply the dependencies directly to the consumer project in the same dependencies block that you have already.
Would any of those approaches work?