Lobby Gold is earned by playing games. A player will receive 12 gold per minute of playtime and is required to be in the game when it ends in order to receive the gold from that game. When returning to the Game Lobby a message in chat will tell you how much Gold you have earned. You can check how much you have accumulated at the Vault Vendor in the Lobby
Lobby Gold is used to buy various dwarf upgrades. The upgrades are bought from the Loadout Vendor in the lobby, or from typing /loadout in chat. A future update will also allow players to buy Hats with their Gold, these will be very expensive and are meant for players who have already bought all the Loadout items.
Completing the Weekly Quest gives you a Gold Bonus of 4,500 Gold. The Quest is to complete one game of DvZ and it resets at Midnight on Thursdays.