Gai lan is similar to "common" broccoli, but has a longer stalk and a more delicate taste. In reality it is neither a type of broccoli or kale, nor from China. It is believed to have originated somewhere in the Mediterranean but it eventually migrated to China where it is most well known. The Chinese people who immigrated to the U.S. brought the vegetable with them and so it's popular here in Asian grocery stores.
Gai lan is crunchy when raw and tender when cooked. It's delicious with meat and poultry dishes, or just as a simple side dish. It can be blanched, then stir-fried. It is not bitter flavored but it is used similarly to broccoli raab. In fact the leaves are more similar to collard greens.
If you don't have, or can't find Gail Lan you can substitute one of these alternatives: