The Hidden Market for Gunship Battle is a store where you can obtain planes that are 

Tier 3 or above, expendable items, Dollars, and materials needed for plane crafting.

Here's how to use the Hidden Market.

■ How to use the Hidden Market

[LOBBY] -> Click [Hidden Market] icon on the left -> [Draw x1] or [Draw x10]

※ Every time you draw, the FREE gauge on the top left will fill up. Once it's full, you can draw once for free.

※ Using [Draw x10] will give you at least one tier 6 or higher plane.