No, it has nothing to do with his donating his salary. He paid $0 in income tax for 10 of the last 15 years before he became president. Capital gains would be part of income tax.Without having looked into it at all, could there be an argument that Trump is paying a small amount of income tax because he was not actually paid a salary in that period? If his earnings come from owning companies and other assets, shouldn't he be paying capital gains tax and not income tax on that?
Again, I know nothing about the US tax system or Trump's activities in that period. Just a thought.