The meat on the bones is the whole story. Pork neck and spine meat is fatty, around 220 calories per 100g once it has simmered in its own fat, and a single bowl carries about 150g of it if you pick the bones clean. That is 330 calories. The potato is 80, the greens with perilla powder about 60, the broth 80. Nothing exotic, just a lot of fatty pork.
The 2-person pot at a proper gamjatang place is a different animal. It comes with 6 to 8 bones, more potato, and the table almost always finishes with bokkeumbap fried in the leftover broth. That fried rice at the end is about 450 calories per person on top of the stew, and it is the reason a gamjatang dinner can quietly hit 1,300.
It is also a 3am food. Ppyeo-haejangguk at a 24-hour place after drinking, with rice tipped in and a soju finish, is closer to 1,000 than 600. The protein is real (about 40g) and the bowl is filling, but hangover eating is not where anyone is counting.