Are you trying to execute the gradle tasks through the Terminal? You would first need to install gradle by following the install instructions on Gradle.org. Then, navigate to your project folder and run your desired gradle tasks.
My-Mac:~ user$ cd /Users/user/Documents/eclipse-workspace/forge-1
My-Mac:forge-1 user$ ./gradlew
My-Mac:forge-1 user$ ./gradlew build
My-Mac:forge-1 user$ ./gradlew runClient
It shouldn't be necessary to do this though as all the gradle tasks can be executed through Eclipse (I would recommend you stick with this since it seems you are a beginner). If you have properly imported your project folder as a Gradle Project, all gradle tasks are available in the "Gradle Tasks" tab in your workspace. Use these for when the Getting Started documentation tells you to run those tasks.