> You own assets worth of 50 billion dollars. Assuming there is no capital gain or loss and that your return on investment is 5%, you should be making 2.5 billion USD annually. Half of that would normally need to be paid as income tax to government. That's around 1 billion dollars every year.

You're assuming that gain in wealth translates to taxable income. It doesn't (necessarily).

Suppose that you buy $100 worth of microsoft stock and the price rises so it's worth $150 a year later. You don't owe anything on that $50 increase in value. (You don't owe anything until you sell.)

However, if you put that same $100 into a bank account and received $10 in interest over the same year. You owe taxes on that $10.

I'd guess that a large fraction of BillG's investments are of the form that he doesn't pay taxes until he cashes out, either for consumption or to make different investments.

If so, BillG's taxable income can <$100M/year while his wealth is growing by $2.5B/year.

I suspect that BillG taxable income was under $5m/year during most of the years while his wealth was exploding (due to the increase in the value of his microsoft stock).