Mmmm....if Hollyhocker is owner of holly_blackwell tree on Ancestry, then it seems a lot of this is already known and for my part anyhow, I have just been repeating their research.
Hollyhocker, your offer of posting relevant research already undertaken by you might be worthwhile to avoid duplication of effort.
Also, given Gertrude was young enough to have one or more children after 1930, have you considered DNA, which always has the potential to throw up matches that could lead you back to her as well as potentially help you discover who your grandfather actually was?