It's a part of Cheat Sheet for common Dagger 2 modules.

GsonModule.java

/**
 * Created by sc0rch on 30.06.16.
 * GSON module for Dagger 2.
 */

import com.google.gson.Gson;
import com.google.gson.GsonBuilder;

import java.util.Date;

import javax.inject.Singleton;

import dagger.Module;
import dagger.Provides;

import ru.your.project.BuildConfig;

@Module
public class GsonModule {
  @Provides
  @Singleton
  public Gson provideGson(GsonUtcDateAdapter utcDateAdapter) {
    GsonBuilder gsonBuilder = new GsonBuilder()
        .registerTypeAdapter(Date.class, utcDateAdapter);

    if (BuildConfig.DEBUG) {
      gsonBuilder.setPrettyPrinting();
    }

    return gsonBuilder.create();
  }

  @Provides
  @Singleton
  public GsonUtcDateAdapter provideGsonUtcDateAdapter() {
    return new GsonUtcDateAdapter();
  }
}

GsonUtcDateAdapter.java

/**
 * Created by sc0rch on 30.06.16.
 * GSON Adapter for Date-type.
 */
 
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import com.google.gson.JsonPrimitive;
import com.google.gson.JsonSerializationContext;
import com.google.gson.JsonSerializer;

import java.lang.reflect.Type;
import java.text.ParseException;
import java.util.Date;

public class GsonUtcDateAdapter implements JsonSerializer<Date>, JsonDeserializer<Date> {
  // Only one locale in example, extend this gist to cover your needs.
  public static final DateFormat DATE_ISO_8601_FORMAT = new SimpleDateFormat(DATE_ISO_8601, Locale.US);
  
  @Override
  public synchronized JsonElement serialize(Date date, Type type, JsonSerializationContext jsonSerializationContext) {
    return new JsonPrimitive(DATE_ISO_8601_FORMAT.format(date));
  }

  @Override
  public synchronized Date deserialize(JsonElement jsonElement, Type type,
                                       JsonDeserializationContext jsonDeserializationContext) {
    try {
      return DATE_ISO_8601_FORMAT.parse(jsonElement.getAsString());
    } catch (ParseException e) {
      throw new JsonParseException(e);
    }
  }
}