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
