I saw a number of people with gray robes and shaved heads while I was in Seoul recently that looked like this:
Is this a religious thing? They had Republic of Korea passports, for what that's worth.
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They are almost certainly Buddhist monks - the shaved head and gray clothes (apparently called "gasa") are a mark of their ascetic style of living.
The gray robes worn by a monastic declare that one is a practitioner, and represent the spirit of no belongings, letting go of all worldly desires.
He is Buddist. Buddists in Korea wear grey and red robes, despite of the most asian countries where the popular color is yellow and orange.
He's a buddhist monk. If he speaks english, say hello and offer to buy him his dinner (don't worry, its not demeaning, buddhist monks live on the kindness of strangers by vow) and you'll get a fascinating conversation, and maybe even a bit of enlightenment!