Would Bill Gates personally fund a UBI? No. The claim wasn't is Bill Gates rich enough to personally fund an American UBI. He would still see his taxes rise considerably and would fall within the tax net to fund a UBI. Those taxes would reach far down the income and wealth ladder, but as long as your UBI is below GDP per capita, it's feasible and a nation could fund it, if it so chose.

While I support a universal income gradually reaching up to (and eventually perhaps well above) the level at which it could fairly be called a UBI, it is not at all the case that any UBI level up to the pre-UBI GDP per capita is affordable.

A universal income will affect behavior, and through that real GDP; at some levels, it may actually improve GDP by reducing job market friction and better allowing people to retrain, retool, and optimize their contribution and their own returns; OTOH, there's certainly a level at which it draws people out of the labor force in large enough quantities to overwhelm any advantage from reducing friction, crashing real, post-UBI GDP.

Sure but there were over 10 million millionaires in the US last year. If we took just a million a year just from those guys we'd have 30,000 a year per american. I'm not saying we should, but your math sucks.

a millionaire is a person with $1MM or more in net worth. the lion's share of these people have the low single-digits millions, as you would expect. after you take $1MM from them in the first year, they won't have much more to give in the subsequent years. net-worth ≠ income.