Both Gameplay & Gamestation belong to the parent company, Game. Game bought Gameplay in 2004 & Gamestation in 2007:
http://en.wikipedia.org/wiki/Game_(retailer)
The "merger" is probably to cut down operational costs, as Game is at risk of entering into administration:
http://www.eurogamer.net/articles/2012-03-07-game-and-gamestation-having-a-firesale-on-pre-owned-stock