Gamble chest are chests 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.
As of Elona+ version 1.68, a sound will play if you win. Gamble chests have a 1/30 chance of awarding a lesser jackpot worth 20x the chest's base purchase price and an independent 1/75 chance of a greater jackpot worth 50x their price, though you can only win if the rare loot trigger is on. They otherwise drop gold worth about 1/20 of their price. Amortizing this over large numbers of chests, you can expect to get around 40% more gold than you spent on the chests before negotiation.
Prior to version 1.14, gamble chests frequently contained small medals.