I have been working on this for over 25 years, and so has Abb Lynn Gunn, who is the North American Genealogist for the Clan Gunn. Contact him, but I can share this.
The genealogy and Revolutionary War service of Reuben Gunn is very well documented. Descendants of almost all of his children are in DAR and SAR. I am the first from the line of his son Lawson Gunn to have my line verified by the DAR (January 2006), membership # #840322. You can go online to the DAR http://www.dar.org/natsociety/pi_lookup.cfm
to request a "Patriot lookup" for Reuben, and ask if any of the lines go through Abisha. Papers will include genealogy and sources of information that were solid enough to be accepted by the DAR.
You can obtain copies of Rev. War records and Abisha's service record from the War of 1812 at the Tennessee Archives and Library in Nashville, or from the National Archives in Washington, DC.
Reuben Gunn served the Colonial Army and was paid 6 pounds, 1 shilling and 4 pence by Mebane & Nichols for service to North Carolina.
I also have copy of land grant etc.
NOW -- TO THE ISSUE OF BENJAMIN GUNN'S BOOK AND THE "QUESTIONS" RAISED
The name Gunn comes from the Norse Gunni, and the Gunns were the Norse Earls of Orkney -- islands off the northern coast of Scotland (near Skye.) If you ever have the chance to visit, please do so. Gunns from all over the world gather there every three years, and there is a wonderful Gunn cultural center in Caithness. See info at http://www.clangunn.us/
But YES! There is a German link, and it is entirely possible that Reuben was descended from a German with a title. Now -- as nearly as I can determine, and if I'm wrong on his Abb will correct me -- the only error in Benjamin's book is the particular title of Reuben's ancestor. He wouldn't have been the son of a grand duke -- that would have put him line for the throne. It was lesser titled nobility.
Research has found Sir William Gunn, who left Scotland for Germany (following the Jacobean War) and married into German nobility, and was later awarded a title. Although no heirs of Sir William Gunn have yet been proven, the link between the Scottish Gunns and the German title has been made. It is also possible that Sir William Gunn as an uncle, rather than father, because no male issue of Sir William Gunn of Germany has been proven, although research is ongoing in America, Germany, and Scotland.
Through participation in the GUNN DNA project, descendants of Reuben Gunn in America were conclusively proven to be related to Sir William Gunn's line, and are genetically linked back to the earliest Scottish Gunns, including The Crowner.
Contact Abb Gun for more on this link. firstname.lastname@example.org
I have put years of work into Reuben and gladly share documented information. contact me at email@example.com
for more info.