Earlier quoted context omitted.
I don't follow this line of reasoning. It's his money, not taxpayer money. If he simply held on to the money, he would not owe additional tax on it either. Perhaps it will eliminate his personal tax bill for 2018, but his yearly income is far less than $1.8B, and his tax bill is only a portion of his yearly income... the tax implications of this gift are only a small portion of the amount.
Unless he is an idiot, he donated aprecated assets like stock. If he sells the stock he needs to pay taxes on that stock. That’s true even if he buys something by directly handing someone stock. Even if he had 1.8 Billion in cash and ‘only’ got to deduct from his current tax bill that still means the US population is giving up money to support his cause. Taxes are not about the government. They are a shared burden by…
He would need to pay tax on the capital gains. We have no way of knowing what the stock's cost basis is.
For sake of example, let's say the $1.8B had a cost basis of $1B. In theory the tax bill would be $160M. While that's a decent chunk of change, I definitely wouldn't describe it as "a large chunk" of the $1.8B being "subsidized by tax payers".
Also keep in mind the ultra-wealthy have all sorts of tax avoidance schemes anyway, and regardless of what he spent this money on, he is likely paying a much lower taxation rate than you or I (or the example above). That's a separate topic though.