答案：that is 或者 that's 或者 which is
1、这里考察的是定语从句的用法.
2、由于先行词是music,是物,所以关系代词可以是that,也可以是which.
3、that或者which在定语从句中充当从句的主语,即：【that/which】 is gentle with great lyrics.
4、gentle 是形容词,所以宾语从句还缺谓语动词,再者,主语是that或者which,即：前面的the music,所以谓语动词用is.
5、综上所述,正确答案是：that's或者that is,或者 which is.