...
(One person of every 300 owns a million dollars or more?!? That figure surprises me quite a lot.)
Sounds about right... private financial wealth typically includes not only cash on hand and savings, but retirement instruments - money market funds, listed securities, etc. Its around 5 million households in the U.S. (note, the U.S. also leads the pack in number of 'millionaires' - but note how this measure is tied to investments...)
It
doesn't include residences, personal businesses, or luxury goods. Nor the
debts associated with them. If those 5 million households had to come up with
1 million net cash most could not.
Keep in mind also, that GDP is an economic measure - not a direct 'wealth' figure.
GDP relates to purchasing power/trade - not individual salaries, savings, or assets. The world GDP (nominal U.S.) is ~ $70 trillion, while the private financial wealth is ~$122 trillion.
And - that later doesn't include company wealth - ala Apple's > 60 billion war chest, etc - which is significant in GDP.
You are relating apples to oranges here (pun) - you asked a question about counts of 'millionaires' - it is incorrect to assume a
direct correlation between accumulated personal financial wealth and GDP per capita. There are quite a few countries that exceed U.S.'s - but their count of millionaires is lower. This relates to factors like overall higher salaries/cost of living, top heavy wealth, and economy including exports and imports, health care - not to mention how much government plays a role in investments.
You
could use GDP and the financial wealth information provided to come up with a model based on a snapshot of
our world ... but its application to Traveller (even assuming one adjusted for its mixed use of 1960-mid 1970 nominal earth related to 2d6

) would be quite, uhm, challenging across TLs and within the limits of UWP stats.
Traveller doesn't have stats (to my knowledge) related to actual overall 'wealth' of a planetary population. Consider, an individual low pop world is probably more likely to have a higher percentage of super wealthy individuals whose monies do not move in the local economy - as their GDP based on pop and the x0.512 - x1.92 multipliers of Striker are not going to support them doing so...
I can see coming up with a random stat that took into account trade codes, law level, government type, TL and pop to come up with some type of wealth distribution model. But it would be tricky and of questionable value.