Update Gradle.gitlab-ci.yml to reflect different handling of Gradle build tool...

As far as I understand chapter Cache vs artifacts it would be a better fit to handle Gradle tool and execution "artifacts" (like 3rd-party dependencies, or Gradle itself, especially if using Gradle Wrapper) as content to cache (especially to cache them between executions of pipeline).

Only artifacts of the building process itself (like compiled classes, deployment artifacts like JARs or Docker images) should be considered as GitLab CI artifacts to push them from stage to stage during a single pipeline run; and not between different executions of pipeline.