There are classes used from Gson internal API
com.google.gson.internal.Streams is used in com/microsoft/graph/serializer/ODataTypeParametrizedIJsonBackedTypedAdapter.java
com.google.gson.internal.bind.ReflectiveTypeAdapterFactory is used in com/microsoft/graph/serializer/FallbackTypeAdapterFactory.java

This is also the reason why this library cannot be used in the OSGI environment as starting from Gson 2.6, the com.google.gson.internal packages are no longer exported in the OSGi bundles.