Is ghusl absolutely required for returning to Islam, or is it only required if the apostate did something that necessitates ghusl?

Ghusl (ritual bath) is not obligatory for a disbeliever upon embracing Islam, unless they committed something during their disbelief that necessitated ghusl. Performing ghusl as a precaution is better. This issue, like other issues, is one about which scholars have differed, and the common person should follow whichever scholar they trust in their knowledge and piety.