I have a possible John Gaby(that is how it was spelled by the researcher in Tn.) The researcher was not sure but placed John Gaby in the family of Isaac Gaby and Magdelena (Frantz.)John, which you indicate as possible father of William Martin, was noted as a possible twin. It was not indicated to whom. The children of Isaac Gaby and Magdelena Frantz are listed as Martin F born 1796. CF born 1810 and Isaac P born 1806. This family sheet was found in the Greenville Tennessee Library.I cannot say for sure that these are the people you are looking for to connect your line. The Gaby family is very elusive trying to research from far away.I live in New England.Check the other listings on Gaby done by Samuel Gray.