Gamble chest are chest you can find in dungeons, as well as buy from NPC vendors. They generally contain a small amount of gold, but have a small chance (1 of out 75) to hold large amounts of gold (ex.: 11k gold from a level 3 chest, 62k from a level 18). Even if the player does not find much luck in opening them, they should still be bought and opened for Lock Picking practice.
Prior to version 1.14, gamble chests frequently contained small medals.