Mustard greens (gat)
Pungent, peppery leaves of brown mustard, in Korea often with reddish-purple leaves, used mainly for kimchi.
Gat is a Korean variety of brown mustard with a strong, mustardy heat that softens and rounds out during fermentation. Fresh mustard greens are found in Asian grocers, sometimes at farmers' markets or as seed to grow yourself, though the red Korean variety is rare.
Substitute:
Green Chinese or Japanese mustard greens from an Asian grocer come closest in flavour, only the reddish colour is missing. Rocket matches the heat but is more tender and collapses faster when pickled.