"그게는 커피입니다." 그게는 is wrong. It doesn't exist in the language. The subject of a sentence is marked by either 은/는 (topic marker) or 이/가 (subject marker), and these sometimes contract to create new forms. In both, the first one (은, 이) comes after a consonant-ending syllable(받침), and the second one after a vowel-ending one. So, - 그것은 커피입니다 = 그건 커피입니다 = THAT IS coffee (meaning works around "what that is"). - 그것이 커피입니다 = 그게 커피입니다 = THAT is coffee (meaning works around "which one" is coffee). - 어제는 화요일이었습니다 = 어젠 화요일이었습니다 = YESTERDAY WAS Tuesday. - 어제가 화요일이었습니다 = YESTERDAY (and not any othe day) was Tuesday. Note the contractions 그것은 -> 그건, 그것이 -> 그게, 어제는 -> 어젠, 어제가 -> (no contraction). You can see 그게 already has the subject marker 이 in it, so additional 는 cannot attach to it because the subject can only have up to one such marker.

그것은 커피입니다 is correct.