If I remember my Spanish linguistics correctly, the Z in Spanish was introduced during the Moorish times (roughly 700 to 1490 CE). And I do think that it was the Jews at that time that introduced the actual letter (and suffix of adding -ez to Spanish names).
This does not mean that all Gomezes and Hernandezes are of Jewish origin. It means that the sounds and letters were of Hebraic/Ladino origin.
