Technically it isn't a tax (rather a social security contribution), although it behaves a lot like one, which is why it could be considered a not very stealthy tax. But that was just an example we can ignore that example and move on, others claim Student Loan repayments are a 'stealth tax', which I disagree with too but consider to have more merit than classing gambling as a stealth tax.
I can't countenance having gambling as a 'tax'. It's nonsense in my eyes, we may as well call anything that costs money a tax. I understand that statistically lower income households are more likely to buy lottery tickets etc but really that still doesn't make it a tax because it's all optional. You can't refuse to pay income tax but you can refuse to play the lottery. When I was poor I paid zero 'tax' on gambling but I did pay income tax - I didn't start gambling until I was not poor. Are football tickets a tax on the poor as well?