Hi, Joshua—"That of" means "the GDP of." China's GDP (i.e., the GDP of China) is being compared with the GDP of the U.S. If the meaning is to be preserved, you can't omit "that of" without making other adjustments.
When you block a person, they can no longer invite you to a private message or post to your profile wall. Replies and comments they make will be collapsed/hidden by default. Finally, you'll never receive email notifications about content they create or likes they designate for your content.
Note: if you proceed, you will no longer be following .