A very marginal one here, but Serpollet were still racing in the Gardner-Serpollet era. All my sources give the same brief info about Gardner - American, Civil War veteran, made fortune in gold mining in Australia.
All I have been able to add comes from a court case reported in New York Times, April Fool's Day 1905 - in Paris, Carrie Wisler or Swain sued 'the well-known American financier and race-horse owner' for $200,000 and claimed to be his wife. Gardner won, and she was forbidden to use his name. No other details. They tell me there were 40 Frank Gardners in the Civil War and there is no easy way to tell how many survived.
Anyone know anything more about this particular Frank?