Both 저 사람 and 그 사람 could be translated as "that person" in English, but only the first could mean "that person over there." The second, 그 사람, would imply a person who is close to the speaker. Though I imagine it would be rude to talk about 그 사람 like they aren't there!

If you still have the doubt. 저 is for that, 것 is thing, so combined 저것 (or 저거) is that thing. Hope it helps :)