After verbs of motion like идти we normally use acc (going to a place). For example, - я иду в театр / в ресторан / на выставку. After в/на both acc or prep are possible, the choice of the case depends on whether you're in tha place or just going there. Compare: - ты где? - я в театре. (location) - куда ты идёшь? - я иду в театр. (direction)

you know that гости is 손님들 in Korean. But in this case "идти в гости" it is just used in meaning of place or purpose. So you use it in original form. If you say "идти в гостей" it sounds like :go inside of people 손님속에 가다. And if you say "идти в гостях" it sounds like you have come to someone's place and you are just walking around there (in the house for ex). "идти в гости" is an idiom and you should simply remember it.

Why do we say "идти в гости" and not "идти в гостей" or "идти в гостях"? (Идти куда? Идти во что? ) В гостИ. "в гостей" is incorrect. You can say "для гостей", "от гостей", "один из гостей". (От кого? от чего?) "идти в гостях" is also incorrent. You can say "сидеть в гостях", "усомниться в гостях" (В ком? в чём?)

В + a professional or other group (note that what appears to be the nominative plural in these constructions is historically an accusative): Он не годи́тся в музыка́нты He is not cut out to be a musician идти́ в го́́сти to go visiting произвести́ в офице́ры to commission