Thank you for your prompt reply. I've been conducting more research in the last week to provide additional useful information. I checked Wisconsin birth, marriage,and death records - no mention of a Grace Guynup or Grace Dake. An online cemetary search showed that a couple of the "Guynup" family trees seem to incorrectly assign birth, marriage, and death dates and information to what they think is a Grace "Guynup/Dake". In one instance Aaron Ruben Hyatt marries a Grace Mary Gray (according to the family trees he marries Grace Guynup); in another instance there are records of a Grace Hyatt - maiden name Hyatt - (according to the family trees this person's birth/death dates are assigned as belonging to Grace Guynup). The 1880 federal census shows that Frances (Dolph) Dake and her husband Robert as having two children, neither of which is named Grace, or born before their marriage. If there was a child born from from Frances' marriage to Augustus Guynup, it must have died before the 1880 census. It doesn't seem plausible that she would have been given up to be raised by another family. In the 1910 federal census Frances is listed as having 10 children, 8 of which were still alive. So, the mystery remains.
On another note, I can see that Augustus Guynup was probably the trouble in that first marriage to Frances Dolph: my research has shown that on several occasions (once for a period of 11 years!)he deserted my great-great-grandmother and her children (without sending any financial support). Reportedly he had problems with alcohol. I have a slim thread of evidence that he may have even been married to someone else prior to Frances Dolph. One of this brothers was brought to trial for attempting to derail and rob a train, and later this same brother is tried for bigamy! Not a branch of the family tree to be proud of, but there you have it.