Android undefined is not an object evaluating custom module - React Native

I am trying to learn how to write a react-native module, and I am getting stuck with this error:

Here is what my code looks like:

Module.java (../android/src/main/java/com/reactlibrary)

public class RNGifMakerModule extends ReactContextBaseJavaModule {

  private final ReactApplicationContext reactContext;

  public RNGifMakerModule(ReactApplicationContext reactContext) {
    super(reactContext);
    this.reactContext = reactContext;
  }

  @Override
  public String getName() {
    return "RNGifMaker";
  }

  @ReactMethod
  public void alert(String message) {
    Toast.makeText(getReactApplicationContext(), message, Toast.LENGTH_LONG).show();
  }
}

index.android.js

import { RNGifMaker } from 'GifMaker';

const onButtonPress = () => {
 RNGifMaker.alert('Hello World');
};

package.json

"dependencies": {
    "react": "16.0.0-alpha.6",
    "react-native": "0.43.3",
    "GifMaker":"file:../"
},

index.js (../)

import { NativeModules } from 'react-native';

module.exports = NativeModules.RNGifMaker;

I have run both npm install and react-native link.

I have (for the most part) followed this tutorial.