BiTGApps is a custom version of gapps package. It is a minimal sized package. BiTGApps packages for supported Android Versions does not go beyond 90MB in size. It is an optimized gapps package. i.e; It comes with optimized APKs, Battery optimization for all system installed...
NikGapps project started with the goal to provide custom gapps packages that suits everyone's needs. A package that a user needs but cannot find and ends up setting up the device installing more apps and removing unwanted apps manually.
Google Play Services is a proprietary background service and API package for Android devices from Google. Google Play Services provides core functionality like authentication, contacts synchronization, and location based services.