Nether Gold adds gold ore to the Nether. But when mined, it drops more than standard gold ore. It will give you gold nuggets that can be crafted into ingots but it also has a chance of giving you fiery gold dust.
That dust can be smelted into Fiery Gold Ingots. These can then be crafted into Fiery Gold tools that have a slightly higher attack speed/damage than regular gold.
And you can also craft a stronger gold armor on par with that of Iron Armor!