You have several thing to determine before you try to answer the question of who is Gilbert S. Gay's mother.
First, which Gilbert Gay do you mean? There are two that have been confused for many years. Gilbert S. Gay is not the son of Allan Gay at all, but is the son of Joshua Gay, who was Allan Gay's brother. Gilbert (no middle initial) Gay who was the son of Allan Gay is a different man. Where did your Gilbert live and when did he die?? Gilbert S, Gay died in Coosa County Alabama in 1857. Gilbert Gay lived much longer and died in 1881 in Carrol or Coweta County Georgia. Gilbert Gay (died 1881 GA) was the son of Allan Gay and Abigail Castleberry. There is no proof at all that Allan Gay ever married Celia and no proof that she was anybody's mother. I am a descendant of Gilbert S. Gay (who died in Coosa County AL in 1857). There are several researchers who are descendants of the other Gilbert Gay. When you figure out which man you are descended from look at my home pages at www.lakemartin.net/~ehend