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

ㅅ ending is often omitted in Korean.

무엇 -> 무어 -> 뭐 이것 -> 이거 이것이 -> 이거+이 -> 이게