Only Gold Nuggets earned from harvesting count toward this achievement. Unfortunately nuggets the farm hands collect do not count towards the tracker and previous nuggets earned before the update do not count either.
There are many different ways you can go about this achievement you can focus on crops, trees, animals, and flowers. Your chance of receiving a nugget increase the higher the level of what ever you are harvesting is. I personally had the best luck harvesting flowers, I planted the entire far right side of my map in Daffodils that can be harvested every hour or so. I unlocked the achievement in about a day doing this.