Koreans avoid using pronouns in the first place, and that's probably the hardest part for most westerners. It's a context oriented language while the Indo-European languages are contents oriented ones. https://www.youtube.com/watch?v=4X4h-NN_RdQ BTW, 그이 literally means "that specific someone", and mostly means "my husband". And 걔 is the short version of 그아이 (that specific child/kid), implying this third person is lower ranked than the speaker. When speaking Korean, wrong use of pronouns is probably the worst mistake you can make, offending people. Be extremely careful, or better, avoid using them altogether. https://www.youtube.com/watch?v=ctAXnki3w0Y Above is a scene where the the girl deliberately use improper pronouns to provoke her older sister. And a lot milder after five years: https://www.youtube.com/watch?v=2QtSN8h90DI

Personal pronouns in Korean are rather complicated. Most third person pronouns are compound word made up of 그(that) and a noun, as in 그것(it), 그 사람, 그 여자, etc. There are three kinds of third person pronouns: for children and teenagers, and familiar and formal reference for adults. We have 그 아이, 그 애, or 걔(not 개), which are different forms of the same word for children and underage youngsters. We can be more specific by saying 그 남자아이, 그 남자애 or 그 여자아이, 그 여자애. We call an adult male 그 사람 or 그 남자 informally, and 그분 or 그 남자분 formally or in their presence. An adult female is referred to as 그 여자 in plain language, and 그 여자분 in formal language. In a book of fiction and other similar settings, we use 그(he), 그녀(she), and 그들(they). But we don't use these words in real life, not even rarely. 그 and 그녀 were introduced to Korean a little over a century ago mainly for the need of translating foreign texts. It has taken root in storybooks and other texts but not in real life. Saying 그 or 그녀 in a conversation would be very jarring, like a first heard foreign word. I've never heard it used even once in my whole life. Korean is traditionally averse to referring to people by short pronouns. The underlying idea is to refrain from making frivolous references to adults, and when you do mention them, to do it respectfully using full words like their formal title, relationship in the family, name, etc, rather than a short pronoun. There are also other pronouns and semi-pronouns used only in certain narrow contexts. 그이 is one of them. It is one of the words by which women refer to their husbands.