What is the difference between Gradle and JUnit, and why would I use one over the other?

Please be nice, I’m a newbie.

I will be using one of these as a test harness for Selenium via Chromium. I’m confused whether they are used simultaneously, or if I would use one over the other, and if so, why I might choose one over the other. Thanks.

Gradle and JUnit are entirely different things. Gradle is a build automation tool. JUnit is a developer testing tool. Gradle integrates with JUnit in that it can run JUnit tests as part of the build. Not sure what you mean by “test harness for Selenium via Chromium”.