Gang of Four claims to be the number one card game in Asia. Well, that might or might not be the case, but it’s origins do lie in the Oriental game Choh Dai Di, which is played on a regular deck.
I think Gang of Four is a good version of Choh Dai Di, because it makes the rules easier — Chinese people seem to like complicated rules, all sorts of tricky things like “finish the game playing twos and other players score double points”. Mahjong is chock full with rules like this. Which is nice, but makes learning the game a real effort.
Also, Gang of Four deck is different enough from the standard deck (compare to Tichu, which has a standard deck + four special cards). Six cards of each number also causes more Gangs of Four, which means more fun!
As a side note, it was pleasant to see Pagat update after almost two years of silence. There seem to be more advertising now, but I can live with that. Pagat is such a quality resource it would be a shame to see it die.