这是个省略句,省略了you are,全句是How about are you doing something?谓语显然是are doing。这样的句式口语中很常用,书面语基本不用。再举个例子,A:How are you?B:I am fine. Thanks. How about (are)you?谢谢。
谓语是动词所以doing是谓语
about是谓语