This is no surprise, considering the miserable household numbers that CableWorld posted for GSN in April and May. I assume June was pretty awful, too.
Basically, GSN decided to take the plunge and drop Millionaire from prime time. The show had been eroding, and the network decided that it had to be rested sooner or later. I'm sure they figured on some damage to the prime time numbers as a result, and they figured right.
GSN just hasn't produced a reliable audience generator to keep the numbers up. The poker show does okay in demos, but nothing else looks to be working well in prime time. Regis (or Meredith, if they could do a deal) might return to 8:00 PM sooner than expected, especially if Without Prejudice and Grand Slam don't work.