Wrapper
Look in the root of the project for a gradlew file. If it exists, you can use the Gradle wrapper to execute the build without installing Gradle. Learn how to use it here. If you want information on how it’s generated in the first place, check out the user guide.
If that file doesn’t exist, you’ll need to install Gradle. The user guide explains how to do this generically for all platforms.
That’s not the only way you can install Gradle, though, the following links provide alternative approaches:
Detailed guide on installing Gradle on Windows (blog)
Installing Gradle with GVM (requires bash or zsh)(video)
Installing Gradle with apt-get on Ubuntu (blog)
Installing Gradle with Homebrew on Mac OS X (blog)
Using the command line
The user guide explains in depth how to use the gradle command (or the wrapper, gradlew) to list tasks, execute them, and query for other build information.
IDE
The user guide explains how to use the Eclipse and IDEA plugins to generate the files that you can then open in the corresponding IDE.