I love GPKs! I've been collecting them since the first series in 1985.
One of the best reference sites for GPK is geepeekay.com. Recently on there facebook page, they held a giveaway. I was one of the lucky winners. I got two website logo stickers, Bailed BART from 2014 and a foreign Bukimi Kun card.
Thanks GeePeeKay for the great stickers!