How to Install Gradle Plugin in Eclipse
The best way is to open Install New Software ... and a site located at http://dist.springsource.com/release/TOOLS/update/e4.4. And then you choose Core / Eclipse Integration for Gradle -> Gradle IDE from the site.
I would not recommend you to use Eclipse Marketplace to search and install Gradle plugin. If your Eclipse version is old, for example Luna, it's highly likely that many of the plugins will be unavailable. They all take quite a while to download and at the end of the installation process you may get an error.
If this is not the first installation you should check the configuration after the plugin is successfully installed. The reason is the plugin configuration is stored in the workspace directory such as workspace3.8/.metadata/.plugins/org.eclipse.core.runtime/.settings/org.springsource.ide.eclipse.gradle.core.prefs, the newly installed plugin will reuse the existing configuration if it exists. For example, you may have used your local Gradle installation instead of the built-in but the path is not valid anymore.