I think the best treatment for addiction, as long as that addiction is not so strong as to need weaning or a substitute, would be to go cold turkey with adequate peer/social and other support, in-person, to start.

For an addiction to gaming, a good way to do this could be to go off-grid in a multiple week wilderness program such as NOLS.

After that, it could likely take multiple months before they could recover alone safely, and, even after that, they may need a sponsor or at least a therapist to help for years possibly.