It was at the gate where I told him the news.是什么从句?是主语从句吗?

It was at the gate where I told him the news.是什么从句?是主语从句吗?
此句是改错题。
强调句没有用where引导，应把它改成that，才是强调句！
即It was at the gate that I told him the news.
但本人有个疑问，就是不知怎么解释这个句子不是主语从句？it做形式主语，that从句为真正的主语，好像说得通！