I was told the $20 would be credited at the end of the month following the month it is used. I never got a definitive answer if I still had to be Platinum at that time. I was told you have to be Platinum when you buy the GC, though.
I was going to join because I wanted to get a Hyatt, but then the one I wanted became available thru regular channels. So right now I have no reason to join. If they ever have London availability, I will be there!