Serialization of ItemStack works perfectly with gjson, {"type":"DIAMOND_SWORD","meta":{"enchantments":

,"repairCost":0,"hideFlag":0,"unhandledTags":{}}}

But deserialization do not work. But it worked in 1.7.9. I guess its becuse of this change.

Object raw = args.get("meta");
            if (raw instanceof ItemMeta) {
                result.setItemMeta((ItemMeta) raw);
            }

Theres no code to check for HashMap and convert it into ItemMeta. In a later version of spigot, version 1.7.9 there was code for this.