Its shares will start trading on London's Alternative Investment Market and Dublin's Enterprise Securities Market on Monday.
The placing will give GAN an initial market capitalisation of €90m (£75m) when it makes its stockmarket debut.
Institutional investors including Blackrock and Artemis are among the new shareholders. As part of the placing, 63pc of the issued share capital will be subject to a 12-month lock-in followed by 12-month orderly market arrangements.
GAN's original shareholders have sold portions of their stakes as part of the flotation.
Mr Smurfit's stake falls from 16.6pc to 13.3pc. His uncle, Michael Smurfit, sees his stake cut to 9.54pc from 15.5pc.
Michael Smurfit is also a non-executive director at GAN. Non-executive chairman David O'Reilly will see his 6.6pc stake reduced to 4.09pc.
Betfair co-founder Andrew Black sees his holding reduced to 9.9pc from 14.1pc. Tony Smurfit, the chief operations director of packaging group Smurfit Kappa, will own 8.5pc, down from 11.5pc.
IIU, the investment vehicle owned by financier Dermot Desmond, will have its stake reduced to 2.19pc from 3.6pc.
Dermot Smurfit said that there had been strong institutional interest which resulted in the offer being oversubscribed.
"We are pleased to welcome quality, international institutions as shareholders on admission and believe that this reflects the strength of our investment case and prospects," he said.
GAN has partnered with Betfair to provide Trump Plaza Hotel and Casino in New Jersey with its internet gaming system, to offer regulated, real-money online gaming from December 31.
In Connecticut, GAN has licensed its internet gaming system to the Foxwood's Casino Resort to offer simulated gaming in anticipation of online gaming being legalised in the state in 2015.
Earnings before interest, tax, depreciation and amortisation at GAN hit €4m in the first half of 2013, compared to just €500,000 during the whole of 2012. Turnover is expected to reach €30m this year compared to €20m in 2012.