I don't think the typhoid blankets your referring to (at least that's what I assume you meant by "gifts that would harm them") were specifically given on the "first" thanksgiving, but you're absolutely right that it happened. Even if the first thanksgiving was 100% as advertised (which it probably wasn't) then it was a short-lived and tiny amount of human decency in what was otherwise a straight up genocide. Nevermind that the story was more about the natives helping the settlers than vice-versa, so really we're just celebrating the time our victims were nice to us while we were still getting ramped up on eradicating their people