There are multiple problems in Gladwells' list of History's richest individuals as listed in his text Outliers: The Story of Success.
First, one must understand the term "outlier". An outlier defined by Merriam Webster in the following way:
: something (as a geological feature) that is situated away from or classed differently from a main or related body.
Therefore, Gladwell is defining the richest individuals in history as those who are not typical in mainstream society.
Gladwell states that the only way one can succeed is by honing their skill by dedicating 10,000 hours to it. This, simply stated, forces one to become basically obsessed with the perfection needed to insure mastery.
This being said, Gladwell fails to recognize the aspect of luck in gaining absolute richness. Gladwell focuses completely upon the aspect of nurture over nature.
What Gladwell fails to acknowledge is the aspect of "self-made" and the simplicity of simply wealth happening without the notion of obsessiveness or dedication to honing specific skills.
Without acknowledgement of the miracle of wealth (the lottery, winning a very large settlement, or passing down of "old money"), Gladwell dismisses those of this category as not being rich. Basically, Gladwell only wishes to acknowledge those who pursue accomplishment and, therefore, gain wealth.