The gradle plugin (which contains a bundled version of gradle) should be already installed in where/you/installed/android-studio/plugins/gradle so you shouldn't need to manually download it. Depending on the version of Android Studio, that last directory may be where/you/installed/android-studio/gradle/gradle-x.y.z (x.y.z is the version, so check your filesystem for the exact path).
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.
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