I reject both of these assumptions. There is no accountability for him in giving away this money, and there is no democratic input into where the money goes. Additionally, he uses his money and his prestige to launder his reputation through favorable media platforms with fawning interviews that never critically examine anything he does.
(Additionally, implicit here is the assumption that government spend is inefficient and private industry is efficient, which is a common right wing trope and again is an assumption I reject.)
We should not and cannot let Bill Gates (or any billionaire) off the hook just for using his money for presumably good causes if we hope to continue to champion the working class and labor in general. The wealth Bill Gates has accumulated is on the backs of the system he has operated in and the labor of those who participate in it; these people should have a say in the use of that wealth, and the mechanism for that is to tax him.
(We can have a separate discussion on the amount of foreign aid that should be funded with US taxpayer dollars.)