Being told project is not using the gradle build system when it is
android, android-studio, gradle
Solution
The android studio support two "build mode". one is "legacy" and one is "gradle based". If you are using an very early version of android studio, probably you are still using the legacy build mode. You need to re-import your project using "gradle mode" then this warning will be gone.
To do so, select "file/import project", select your project root directory, and in the next dialog choose "Import project from external model" and select gradle. Then, you may choose "gradle wrapper" or use your own gradle distribution (1.8 for now[12/2013]).
Problem
When I open my project in Android Studio, I always get this. Back when Android Studio first came out, I went through this process so the project should have the Gradle build system. There is a build.gradle file in the root of the project. I can still build and run the project, but I don't understand why it always says this since the project should use gradle build system. Any help with this would be great.