"Coming from Korea is tough."
Translation:한국에서 오는 게 힘들어요.
Roughly speaking, it is an abridged, conversational version of 것이 (imagine use of "G'day" versus Good day as it is quick and simpler)
것이 = 게, 거
한국에서 오는 것이 힘들어요 = 한국에서 오는게 힘들어요 =한국에서 오는거 힘들어요
For creating your own sentences in speaking and writing, I recommend you to use "것이" rather than 게, 거 as usages for 것이 are more diverse both conversational & written.
Usually, songs and poems prefer shortened versions as such obviously