Motto: EXCRETUS EX FORTUNA (Shit out of luck)
The Gamblers' Guild House is built on the Street of Alchemists, opposite the Guild House of the Alchemists. Given that the Alchemists blow their building up periodically, visitors to the Gamblers sometimes ask why they choose to remain sited here when there is such a high chance of being blown up. They are politely asked if they read the sign on the way in.
The panels on the gamblers' coat of arms (a sabre, an octagon, a turtle, a crowned A, a sceptre, a chalice, a coin and an elephant) represent the eight suits of the classic Ankh-Morpork pack of cards.
Presidents are chosen (how else?) by the draw of a card. The current president is Scrote Jones, although a leading member called Doc Pseudopolis (named on the basis that one should never play cards against a man named after a city) represents the guild in the Rats Chamber in Feet of Clay.
The Guild's chief function is to regulate the use of marked cards, shaved dice, and other tried-and-true means of tilting the odds in a gambler's favor. Not banning them, mind, but ensuring that these cheating methods meet with Guild standards: if two Guild members play against one another, their respective tricks should cancel each other out, and victory defaults to the luckiest and most skilled. Non-Guild players are unfair game.
Gamblers revere but never call on The Lady, as it is well-known that she deserts those who ask for it, but often unexpectedly aids those who don't look for it. A sub-sect of gamblers who really didn't get it set up a religious cult worshipping The Lady. They were all broke and/or dead within weeks.
Coat of Arms by Matt Smith