In such types of gambling as roulette or slots don't have to think at all, and therefore gambler can be in almost any state, either under the influence of alcohol or sober - this will not change the outcome.
But in fact, there is a variable that will certainly change the outcome of the day in which the gambler decided to play, this is the bet and the desire to continue playing. It has been proven that drunk people do not want to stop, the ability to say a strict "stop" to themselves is inherent only to sober people.
And a person under the influence of alcohol will most likely increase the bet uncontrollably.
It is called dis-inhibition and it is scientifically proven to be associated with the use of substances, alcohol being one of them. People always feel some level of inhibition or caution when comes to gambling, that is natural, as nobody desires to lose their money to the casino, but as soon as alcohol kicks in and inhibitions are gone it is easy to lose awareness on the value of money and what one is actually doing with ones budget, money suddenly losses all value to the drunk gambler.
That is why if someone decides to drink and gamble at the same time it is always advisable to restrict liquidity and only carry money one is willing to lose to one's drunkenness and bad luck.
Devote muslin would undoubtedly say this is just a proof of the importance of the words of their prophet and the messages left behind in the koran about the dangers of both alcohol and gambling. But that is up to one interpretation, the three biggest religions on the world do not necessarily condone gambling, they just have varying degrees of tolerance to it.