I have an android project that I can import to android studio. It opens but refuses to run. The file contains 4 folders (.idea, gen, java, res) and 2 files (AndroidManifest.xml and project.apk). I can't seem to run the app. When I import it, Android studio tells me "This project does not use the Gradle and migrate project to gradle.
Yes, Gradle is required for apps developed in or migrated to Android studio.
Android Studio’s build system is based on Gradle and uses build configuration files written in Groovy syntax for ease of extensibility and customization.
Since the whole build system is based on Gradle, you're stuck using it! Not a bad thing, Gradle is pretty great.
I have migrated quite a few projects to Android studio, for the most part it has been quick and painless.
There's a nice guide to help you migrate to Android Studio over at the Android development site: https://developer.android.com/studio/intro/migrate.html