I suppose gender is probably inferred from the user's browsing habits--ie, Quantcast has a record of your IP address across the various sites (like mine) that use its tracking cookie, and if you primarily visit certain types of sites, it guesses you're a certain gender.
In which case its predictions are kind of self-fulfilling, but if they can cast a wide enough web--and I think Quantcast probably does--then the predictions can be reasonably accurate, at least for marketing purposes.
I'm almost absolutely certain Google does the same type of thing--and Google has a greater reach than Quantcast--thus my surprise that they don't show it to you in Analytics.
I got into Quantcast when I found that an ad network I wanted to sign up with used Quantcast's report of my site traffic as pretty much the main benchmark for admittance to their program; my traffic was too low for Quantcast to have formed their own estimate, so I caved and installed their tracking cookie on the site (and thus am now "directly quantified"). The ad network then accepted me ("your traffic seems to have grown!" :P) and apparently used the Quantcast demographic data in assigning me to a general ad campaign category ("hardcore gaming" :P), but I've kept using Quantcast's cookie because it does seem to give some pretty useful data, at least as something with which to estimate your own performance over time, or to compare with other sites.
One of the webcomics I respect most uses it (Bad Machinery
), for instance, and I've started to notice a lot of other pretty well known sites do as well. Furthermore, I think it's probably fairly reasonable to assume that some prospective advertisers wondering if they should bid directly on your site's Google or whatever (especially the non-Google ones, I suppose, since Google's bidding interface--which I have not seen--probably already provides a fair amount of demographic or at least general market data to prospective bidders) ad boxes will look you up on Quantcast, among other tools, to see if you fit their desired demographic.
The page count and unique visitor numbers I get from it tally very closely with the ones I get (much less conveniently) from my web logs--far closer than Project Wonderful's stats, for instance, although I still glance at those during the day since they update near-real-time, whereas Quantcast's only updates daily.