Question

where do you think he has gone？为什么不加to

where do you think he has gone？为什么不加to

Answer 1

where do you think he has gone？
因为go somewhere 去某地 是有这个搭配的。go是及物动词，直接宾语是somewhere.
所以不用go to somewhere.
Where 是 译文副词作状语。这句话也可以这么问：
Do you think he has gone where? 而不是 Do you think he has gone to where?
原因同上。

追问

那为什么不是has he gone

追答

has he gone 是一般疑问句，回答是/否时用。where 是特殊疑问句，回答地点状语在哪里时用到。

Answer 2

加to是对的。

have / has gone to表示
到某地去了
或
已经去（现还没回来）

例如
We could n't think where she had gone to .
我们想不出来她到哪里去了。

Obedient to his order Suke had gone to bed .
遵照他的命令，蘇柯已经上床睡了。

Lydia would have gone to his room and found the place in disarray .
利迪娅会到他的房间去，发现那里乱七八糟。

p. s. has/ have gone可配to 或不配，
例如
She has gone to the library. 这to不能删。
She has gone home. 这里to要删去。
She has gone to Thailand for a holiday. 已经放假去了泰国（现还没回来）
She has gone. (离开而不考虑去哪里)