Gap alone is usually translated as 罅隙 in Standard Written Chinese or simply as 罅 in Spoken Cantonese. However when "gap" is used with another word, "gap" is often used only as a metaphor. In such situations, the Chinese language and its dialects often demand greater accuracy in the sense of skipping the metaphor and getting right to the point. This is most evident in translations such as gap year (間隔年) or income gap (貧富差距).
口 alone does not mean "gap", but 缺口 (opening) has such connotation in most situations.
口 alone does not mean "gap", but 缺口 (opening) has such connotation in most situations.