When Gary asks "What is right?" he is asking the question e.g. "What state of affairs will help people have more fun?" and not "What state of affairs will match up with the current preferences of Gary's brain?"
I do not necessarily disagree with this, but the following:
and the proof of this is that if you offer Gary a pill to change his preferences, Gary won't take it because this won't change what is right.
... does not prove the claim. Gary would still not take the pill if the question he was asking was "What state of affairs will match up with the current preferences of Gary's brain?". A reference to the current preferences of Gary's brain is different to asking the question "What is a state of affairs in which there is a high satisfaction of the preferences in the brain of Gary?".
Perhaps a better thought experiment, then, is to offer Gary the chance to travel back in time and feed his 2-year-old self the pill. Or, if you dislike time machines in your thought experiments, we can simply ask Gary whether or not he now would have wanted his parents to have given him the pill when he was a child. Presumably the answer will still be no.
In You Provably Can't Trust Yourself, Eliezer tried to figured out why his audience didn't understand his meta-ethics sequence even after they had followed him through philosophy of language and quantum physics. Meta-ethics is my specialty, and I can't figure out what Eliezer's meta-ethical position is. And at least at this point, professionals like Robin Hanson and Toby Ord couldn't figure it out, either.
Part of the problem is that because Eliezer has gotten little value from professional philosophy, he writes about morality in a highly idiosyncratic way, using terms that would require reading hundreds of posts to understand. I might understand Eliezer's meta-ethics better if he would just cough up his positions on standard meta-ethical debates like cognitivism, motivation, the sources of normativity, moral epistemology, and so on. Nick Beckstead recently told me he thinks Eliezer's meta-ethical views are similar to those of Michael Smith, but I'm not seeing it.
If you think you can help me (and others) understand Eliezer's meta-ethical theory, please leave a comment!
Update: This comment by Richard Chappell made sense of Eliezer's meta-ethics for me.