Why did Gechazi get Zoras?

Thank you for your question, Simply put, it was 'mida keneged mida', or a punishment that is 'measure for measure'. Elisha tried to make a Kiddush Hashem by healing Naaman's tzaraat. Gechazi took advantage of the situation by obtaining some riches from Naaman. That act was a desecration of Hashem's name since he made it appear that Elisha was in it for money and was not such a holy man. Therefore, Gechazi was stricken with the same tzaraat as Naaman suffered from.