I did find what I needed to know about Rev. Gustav Niebuhr, from several sources. He served churches in California, in Wright City, Missouri, and in Lincoln, Illinois--perhaps other places. He is indeed the father of Rev. Reinhold Niebuhr, as well as two other sons and a daughter. I have posted the Niebuhr-Niemann line that I share with that family at Rootsweb's WorldConnect project. (Rev. Reinhold is Karl Paul Reinhold.)
I don't believe the Niebuhrs all connect--as a matter of fact I have another Niebur (before spelling was standard) line and cannot connect that family to the Niebuhr family into which my great-grandfather's sister married, although geographically they are only a stone's throw apart. For your study, I'm pasting in below a short portion of what I've written about the Niemann name in my forthcoming book (formatting has been lost):
"With my marriage, the Niemann name daughtered off in the United States. Here I will explore the meaning of the name. Whatâ€™s in a (German) Surname? by JÃ¼rgen Eichhoff appeared in the August/September 1999 issue of German Life, relating to the development of surnames in Germany. Mr. Eichhoff writes:
â€œIt can be said that in the 11th century A.D. there were no family names; by the 1500s they were firmly established in most places.â€
He goes on to tell that names are tied to the German linguistic environment, reflecting the dialect in the area from which they originated, and mentions some technical changes that occur in the various dialects.
â€œOriginal monophthongs shifted to dipthongs [sic] in the south, accounting for names such as Neumann (new man), Neubauer (new farmer), and Mauermann (bricklayer) but Niemann, Niebuhr, and Muhrmann in the north. In central Germany there is an area stretching from the east to west where the euâ€™s became auâ€™s accounting for Naumann and Nauberâ€¦the first element also meaning â€˜newâ€™â€¦â€.
So, we can assume that the first Niemann, arriving just about the time surnames were being acquired, was the â€œnew manâ€ in the community. Inasmuch as new men in many communities might have been assigned that name, Niemanns living in various places didnâ€™t likely come from the same roots.
(It should probably be noted here that names beginning with â€˜Niederâ€™ donâ€™t fall into this category at all; nieder means â€˜lowerâ€™ and refers not to status in life but to altitude.)