기숙사에 오신 것을 환영합니다 = thanks for coming to the dorm.
I don't think "오신 것" is a gerund because gerunds are in the present tense: E.g. "오시는 것" = coming
I don't thinks it's a relative clause because it should be "분" then not "것" "오신 것" = something that came * sounds rude referring to a person as a "thing"(것) "오신 분" = people that came - sounds ok.
So what type of grammtical structure is "오신 것" here?