The union claims some earn as little as £10,000 a year - just £500 above the minimum wage. They also say workers sometimes suffer harassment at the hands of customers.
GMB organiser Michael Ainsley said: "GMB members in casinos have far too long been undervalued by their employer.
"They are fed up with Stanley's failure to deal with low pay in an industry that can't claim to have no money - round £28bn was spent in casinos last year."
Mr Ainsley told the Daily Post the problem of poor pay and conditions in UK casinos was not just restricted to Stanley but afflicted the industry as a whole.
Yesterday Stanley issued a joint statement with the GMB which read: "Following discussions between the senior management of Stanley Casinos and the GMB national delegation, further dates have been arranged to continue the negotiation process regarding annual pay and conditions.
"Both Stanley Casinos and the GMB are committed to working towards a solution to the current issues in a constructive and responsible manner.
"The GMB is in the process of consulting with its members prior to the next meeting with Stanley Casinos' management..
"However, contrary to recent reports, the GMB is not in the process of taking any industrial action."
Last month Stanley agreed to sell its 624 betting shops to rival William Hill for £504m in a deal described as "mind boggling" by one City analyst.
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