Chop it up to separate the stems from the leaves. Gailan is a pretty sturdy green, and will need some serious treatment to make it delicious. Melt some:
in a sauté pan with a tight-fitting lid over medium-high heat. Toss in the stems and sauté for a few minutes until they start to soften. Toss in the leaves.
Toss in the duck fat, and sauté for a few minutes. Put the lid on to trap the steam, and steam the gailan in the duck fat for a few more minutes. They're done when they've softened to the point that they're chewable, but not mushy.
Toss with a bit of kosher salt and serve hot. I generally don't even start this dish until everything else is done because it's so fast. It's rich, slightly bitter and the duck fat and smooth it and make it a spectacular side. And gailan is substantial enough that it even holds up as a leftover (unlike so many lesser greens). Great stuff.