Originally the Gold Box was dropped when battle the fund reached a multiple of 700 crystals. Since Update 1.78.0 it can now appear in a battle with any fund size. The probability is 1 in 10,000.
On February 19, 2016, an option to disable Gold Boxes dropping in PRO battles was added.
Q: Can Helpers or Admins drop Gold Boxes?
A: No, Gold boxes are completely random. They have a 1 in 10,000 chance of dropping.
Q: How long after the notification does the Gold box starts dropping?
A: Since Update 354, You will have 30 seconds after the notification before the Gold box starts dropping. In one of Tanki's V-logs, the developers changed the gold boxes drop time to 10 seconds making it drop faster. Also the gold box was dropped my a metorite instead of a parachute, making it harder to get since all players in the metorite's explosion will be killed.
Q: Where do Gold boxes usually drop?
A: The best place to start looking would be the middle. After you have scanned that area try looking in your or your enemies bases, the edges of the map or other obvious places.
Tankers from the community have put together a list of every place a Gold box drops on every map. This list can be found here
Q: How can I find out where a Gold box is dropping?
A: You are looking for a white square with a golden crystal in the middle. Move your camera to its highest point so you can see as much ground around you as possible. After about 15 seconds since the notification has appeared, bring your camera down to the lowest it can go and start looking in the sky for the gold.
Q: Can Gold Boxes be disabled?
A: Yes. There is now an option to disable gold boxes dropping in PRO battles.