Sometimes they are announced in the News section, but most times not. Here's the alternate way listed by WGT in the Sweepstakes Rules section:
A list of Sweepstakes winners can be viewed on the "Recent Winners" webpage at http://www.wgt.com
and is also available by sending a stamped, and self-addressed envelope
marked to "(This is where you put the name of the tournament you want to find the winners for
)" 180 Sansome Street 5th floor, San
Francisco, CA 94104.
It's a bit cryptic at times, but there you go.