If an android app is using an external library for instance like (Retrofit, EventBus, or GSON), does it have to pay for its service once that same app is published in the Google Play Store (Goes live)?
This would depend on the licence terms of each individual library. For Retrofit for example, I quickly found this: https://github.com/square/retrofit/blob/master/LICENSE.txt which is the Apache licence. It imposes conditions on how you use it, but it doesn't mention that you have to pay the contributors. Other packages may be different - you need to look at each one separately.