That or the.
So this says: The seoul came person - the person who came to Seoul.
Yes, because "그" has the function of making the subject/the noun in the sentence a definitive one; it's essentially replacing "the" in this sentence:
그 서울에 온 사람 That person who came to Seoul = THE person who came to Seoul
There is no equivalent grammatical structure in English. We have to say this phrase using different parts of speech (at the very least). Don't try to calque (verbatim translation) these or you'll get stuck and hesitate to use the structure when speaking later on. This is similar to e.g. "the long-legged person," but in Korean a tense conjugatable verb root and locative noun might be noun phrase modifiers. (I'm not a Korean linguist (yet) so my description might not be the best.) Thus, this is "the Seoul arrived person" and it can become the subject, direct object, indirect object, or the topic (depending on what sounds acceptable to native speakers in the given Korean country).