The 62-year-old actor has candidly opened up on the side effects of addiction, and admitted he "wouldn't wish it on [his] worst enemy".
Speaking to the Los Angeles Times newspaper, he said: "I used to sweat vodka. It becomes such a part of you. My tongue would be black in the morning. I blamed it on the shampoo.
"I wouldn't wish it on my worst enemy, to be in the grip of it. It's hell. And that self-effacing humour? That's just there to mask the inadequacy."
Gary has revealed how he used his own experience as a recovering alcoholic for his role of screenwriter Herman Mankiewicz in new Netflix movie Mank.
He added: "Herman, with that self-effacing humour, he was at lunch, drinking with a friend, who said, 'Why don't you go home sober for once?'
"And he answered, 'What? And have [his wife] Sara throw me out as an impostor?' I did the same thing.
"I would sit down and tell the waiter, 'I'll have a large vodka tonic. And can you bring it now because I'm an alcoholic. I need it quicker.' "
And Gary noted he struggled to quit booze initially because he "romanticised" his addiction and associated it with inspirational artists like Ernest Hemingway.
He explained: "People romanticise it, and even I romanticised it. All my heroes were drinkers or opium addicts, and you get all misty-eyed about these poets and playwrights and actors who were big drinkers."
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Meanwhile, Gary typically feels he can "hide" his insecurity with make-up and prosthetics and feels much more "comfortable" that way, so playing screenwriter Mankiewicz without any transformation, he initially felt vulnerable before eventually finding the job "liberating".
He previously said: "I like a disguise because of my own insecurity. When I can hide, it makes me feel more comfortable. I don't know, maybe it comes back to not feeling worthy.
"I'm coming up to 24 years of sobriety in March, but I remember all the things that made me want to drink, you know?
"So when David [Finscher, director] said, 'I want you as naked as you've ever been, I do not want a veil between you and the audience', it played into my insecurities. He said, 'Trust me'. So you go, 'OK'. And really, it was the best call. Oddly enough, after a couple of days, it was rather liberating."