In writing your letter from the point of view as a person of color or one who embraces the interests of people of color, I suggest that you deconstruct Tom's points.
As you compose your letter, keep in mind that Tom is not the smartest of human beings. You are not going up against a scholar. Nick points this out as Tom trails off in his "theories" on race:
There was something pathetic in his concentration, as if his complacency, more acute than of old, was not enough to him any more.
When you write your letter, you want to take apart his arguments by suggesting how wrong he is.
Tom's first point is his belief about the "dominant race." He opens the discussion with this point: "It’s up to us, who are the dominant race, to watch out or these other races will have control of things.” In your letter, you might want to point out that this is not the case. There are more people of color around the world. The people who would fit Tom's "dominant race" are actually the minority. I would suggest that as a White male, Tom is the minority when we look at the world population statistics. Seeing that there are more people of color around the world, "these other races" already have control of things. Tom is wrong in his initial presumption and bringing this out might be a good starting point.
Tom's second point is that White society has created all that is meaningful in the world. In talking about "Nordics," Tom suggests that any civilization rests in their hands:
This idea is that we’re Nordics. I am, and you are, and you are, and ... And we’ve produced all the things that go to make civilization — oh, science and art, and all that. Do you see?
It might be good to suggest that there are plenty of examples of civilization around the world that were not created solely by "Nordics." You could point to the Chinese dynasties, as well as regal traditions in Africa where civilizations were already created. These civilizations rooted in scientific and artistic advancements were outside of White people. The "Nordics" did not corner the market on the elements that define civilization. Predictably, Tom's argument fades off. It might be effective to mention in your letter that Tom's words condemn millions of people to a subservient position in life. It denies them voice.
As you are writing your letter, you should keep in mind that Tom is a brute. He is a bully. He is not someone with depth in his mind and compassion in his soul. You should make sure that your letter takes a pointed approach towards his philosophy. You can be civil, but the letter should be forceful. Like all bullies, some form of direct confrontation is needed. Part of the ultimate sadness in The Great Gatsby is that the people like Tom win. Maybe your letter could be a voice that suggests he shouldn't.