Either way, it's a modern name, so no biblical links ... I don't know why it would come up in a Catholic book as there isn't likely to be a St Geralyn ... maybe they just didn't think there was a Demon Geralyn either, so it was safe!
Gerald, made feminine by adding -lyn, is how it was done. Lyn, like Ann, was vey popular both as a first name, a middle name and a name element; as you say, Carolyn suddenly became a version of Caroline. I think it's a pretty name, nicer than Geraldine for sure, and much less common than Michelle or Michaela. Your parents did a good job!