In this case, the free gift is the Grey Goose, and it will be available in quantities of one per customer for the next 46 or so hours (so, a little under the next two days).
The cheapest way to take advantage of this offer would be to purchase the 4 Farm Cash package, which costs just 10 Facebook Credits, or $1.00 US. Sure, you may only receive 4 Farm Cash for that price, but you'll also receive a free Grey Goose, which isn't even available to purchase in the store (currently), or receive as a free gift. What's more, even if the Grey Goose was an item in the store, history tells us that the price would be much greater than 4 Farm Cash.
So there you have it - another attempt by Zynga to get you to add to your Farm Cash totals. Will it work? Let us know if you plan on taking advantage of this opportunity before it expires.