http://www.androidren.com/index.php?qa=162&qa_1=gradle-location-is-unknow
老外的一种方案：
The gradle plugin (which contains a bundled version of gradle) should be already installed inwhere/you/installed/android-studio/plugins/gradle so you shouldn‘t need to manually download it.
If you intend on doing gradle development outside Android Studio or want a different version, you can download it separately and point it at that path, but if you only want to get Android Studio working with the bundled version, this path should do it.
我用的是去Android Studio安装目录里找到gradle目录选择的。
原文：http://blog.csdn.net/forlong401/article/details/42804221