Caveats: GDP is no way to calculate wealth, and comparing the total GDP to any one person's wealth is pretty useless.
Let's say the total GDP over the last 40 years was 300 trillion dollars. Also, let's say Bill Gates's wealth is 100 billion dollars (for ease of calculation).
100B / 300T = 0.003 = .3% of 40 years of GDP
Let's say that the average population of the US during that 40 year period was 170M[0].
170M * 40 years = 6.8B person-years of work (PYoW).
Bill gates contributed 40 PYoW to the GDP, which is 0.000000059% of the GDP.
However, he captured .3% of the GDP as current wealth (not including wealth spent during that 40 years)
That means his wealth capture is 5 million times the "average".
That 'tiny sliver' is anything but.
[0] - https://fred.stlouisfed.org/series/LFWA64TTUSM647S