To Thoreau, what did the tax gatherer represent?
I think that Thoreau saw the issue of taxation as a direct representation of what an individual felt. The tax gatherer was a representative of the government. If one did not agree with the government, one should not pay taxes to that government. To pay taxes was something that Thoreau saw as a means to give consent. For Thoreau, the war against Mexico was something that represented a fundamental disagreement. In refusing to pay taxes, Thoreau was following his conscience, embracing a sense of civil disobedience that refuses to accept society as what is and seeks to drive it as what can be. It is here that Thoreau saw the tax gatherer as representing an element that required individual consent and following one's morals in order to validate one's voice and ensure that society and government does the same. For Thoreau, it made no sense that people who spoke of individual action and a sense of heeding one's own sense of identity would willingly participate and engage in something that they found offensive to their own sensibilities. This hypocrisy was something he abhorred and to this end, the tax gatherer represented a moment where his own true nature could be revealed.
Tax gatherer represents the autocratic and dominant state. If you want to understand the importance of a strong government you can use Machiavelli's Prince as an example. In other words we can say that while Machiavelli supports a strong state and ruler, Thoreau thinks that individual's independence is more important than any other thing in a country.