I looked it up. A handful of assorted Pacific Island countries have debts totaling less than $76M. (A few claim to have none. So does Liechtenstein. Never did trust Liechtenstein.) If you can believe Wikipedia, Nigeria's total debt is around $15B, i.e. gigadollars, not megadollars, putting them in the exact middle of the list in raw dollar amounts. But that's only because they’re a huge country--7th in the world, by far the most populous in Africa. Whether per capita or as a proportion of GDP, they're near the bottom of the debt list. (Imagine if your own country’s debt were only $60 per capita. A one-time assessment would pay it off.)
Same as all the other stuff done by the Gates foundation: People don't remember you for being absurdly rich; they remember you for what you did with the money. Think Carnegie rather than Scrooge McDuck.