I see one reasonable option for spending such a large part of income on gambling - if a person has huge savings and current income does not have much influence on his life, then if he wants, he can even spend 100 percent on gambling. All other options look dubious to me, although I am not saying that this is an addiction, just a wrong decision (in my subjective opinion).
Only those person who has a huge savings and current income are for sure who has also the guts to over spend or lets say it is only a part of their excess money. For them it was an excess money but for us or personally speaking for me a 20% of income to allot for my entertainment or gambling activities is such a huge amount. We may take over spending as gambling addiction which is definitely true but for me I look for myself a sign of not having a good sleep at night while keep on thinking of my gambling activities.
The general rule is that, only gamble away the money that you are willing to spend on. If you rely on gambling as a means for a source of income, then chances are that you would lose more money on the process.
Remember that if you rely on a source of income that is measured by odds and risks, then there can be no absolute stable flow of income on your end. You would end up having to bet more than you actually planned for which can result in accumulation of long term debts.