Not possible for a gambling addict to quit slowly. If gambling is a problem, the best is to just quit. After he quits, he can start to gamble again after a year. If he sees that he is still gambling in a way he waste money on it, he can quite again. This is how I stopped my gambling addiction.
But if the gambler do not quit and not able to quit, he needs something like rehabilitation program or medical consultant.
You won't say it is not possible because humans act and react to issues differently. It might not work for you but it might be the best solution for someone else. I have seen drug addicts who are permitted to take lesser addictive drugs to help them gradually quit drugs. Some therapists might recommend marijuana for a heroine or cocaine addict. Gradually it might be reduced to cigarettes until the person fully gains control over his life. So I believe gamblers can quit gradually. Maybe one can gradually reduce the amount and time he spends on casinos and this could help to curb excessive gambling.
Yes anybody willing to have a cold turkey can do that, especially in gambling. But it's not an easy process, the person wouldn't stop immediately. Op's term easier, in my response, is wrong. Because gambling has to do with mental health, and the addict wouldn't have enough control over himself to even realize his condition. Unless he's not deeply into the addiction or in a minimal level of gambling addiction, the person still needs the help of a therapist to get cured of his problem gambling. The procedures you mention above for a drug addict could be used to help a gambling addict, but due to the psychological effects it has on the gambler, the therapist also needs to establish a fresh relationship with the gambler. Because switching addiction is quite very hard for a gambling addict, the therapist needs not to give out straight advice to the gambler. Instead, it's preferable to let the gambler discuss how he feels being a gambling addict. And the pains he's having inside, as an addict.
It's known that the gambler may have gone through difficulties and loved ones going nuts on their new attitudes. Those ill experiences that make the addict feel nobody would be able to help him except himself, need to be shared with the therapist, to help facilitate therapy sessions between the both; therapist and the addicted gambler. Trying to tell a gambling addict what or not to do, doesn't seem effective, because he feels he's actually on the right track and everyone else's responses don't matter to him. So, having close conversations can help him think about the view of the therapist and begin to have a change. It's like a chronic disease, gambling addiction requires lots of time for the person to recover completely, that's why many users in their response said it's not easier to have a cold turkey in gambling addiction. Therapists have to make sure the player doesn't resist help, by not being bossy or making the process more difficult for the gambler. All these take lots of time, and the gamblers need to comply by attending therapy classes.