Error: Could not find or load main class org.gradle.launcher.GradleMain

Basically this means that Gradle can't find your gradle/wrapper/gradle-wrapper.jar.

You have to follow Step 3 and setup environment variables:

Microsoft Windows users

In File Explorer right-click on the This PC (or Computer) icon, then click Properties -> Advanced System Settings -> Environmental Variables.

Under System Variables select Path, then click Edit. Add an entry for C:\Gradle\gradle-3.5\bin. Click OK to save.

Source: https://gradle.org/install#configure

The easiest way, is to simply use gradlew.bat in your project and it will auto download Gradle for you!

I stopped all the gradle daemons by running ./gradlew --stop and the error was resolved for me.