If you are enabling ProGuard to minify an Android APK, then it will be necessary to tell ProGuard to leave the Retrofit and related JSON data classes as they are. Otherwise, you will get class not found errors when running the Android app.
In Android Studio, to add the exceptions to ProGuard, open up the file proguard-rules.pro in the editor. Add in the following lines.
Note: for the last line above, replace it with the path to your classes that correspond to the JSON classes you are using in Retrofit.