Good news! I found out more about Gustav Friedrich WITT of South Bend. I have confirmed that he was indeed the oldest surviving son of August Ludwig WITT (1834-1904) and Wilhelmine WEINKAUF, both of whom died in Pulaski Co., Indiana.
I found his naturalization certificate record on Family Search providing his exact date of birth, 13 Dec. 1861. With this I checked again and eventually found his birth-baptism at Vandsburg parish, Westpreussen, Preussen-- born 13 Dec. 1861, just as his naturalization paper said, and baptized 5 Jan. 1862. The index also showed an older brother who died young-- Adolph Friedrich (1859-1863).
* * I still am hoping someone can help find Gustav's death record (and ways to get his obituary?). I narrowed down the search-- he died between 1942 and 1945 (based on city directories for South Bend). Hope that helps anyone joining in the search. Thanks for any assistance.