This is a simple example on how a multi project could look like and how the gradle-release plugin is configured. In this example we use separate versions for each (sub)project.
Fork the repository and checkout locally.
Then you can test executing the release by
$ cd multi-project-single-version
$ ./gradlew release
On windows you would do
$ cd multi-project-single-version
$ gradlew.bat release
Only releasing the parent/root-project
$ ./gradlew :release
Only releasing one sub-project
$ ./gradlew :subproject1:release
If you have an idea which feature of the gradle-release plugin we could show here, you are welcome to submit a pull request.