I'm not going to go through the litany of SecDef Gate's record but if I did I'm sure I would find many things I both agree and disagree with. That's not the point of my comment though. On the date of the J-20, I don't think Gates was wrong. I have always understood various estimates of the J-20's by the Pentagon and US China Commission (and Sec Gates's statement) to be IOC dates, and the 2020 date was both a respectable estimate at that time and not really that far off from any revised estimates given the changes in assumptions we've had to make since the J-20's reveal.
Yeah if we actually look at his his original words it was actually fairly reasonable — that china would only start to get meaningful numbers of stealth fighters by 2020
The problem is many people interpreted that as meaning we'd only see china come out with a stealth fighter by then, and ignored the entire prototyping process.
Of course, one can ask how many planes "meaningful numbers" actually mean.