it seems like the .gz stuff should be handled at a higher level -- at the MarcReader generally, if not at the actual Indexer to apply to any reader. There doens't seem any good reason to put only in the NDJ reader. 

My inclination is to go in and remove this functionality, until it is a) documented, b) tested, and c) probably implemented at a higher level applying to more than just NDJ reader. Perhaps MarcReader alone to start, or perhaps Indexer applying to any Reader. No?