TrueCrypt does disk encryption so that someone who steals your computer can't read your files. GPG does file encryption so that you can send files to other people without someone spying on you. They don't do the same thing.<p>Your question is phrased argumentatively ("If GPG is so great") for someone who hasn't understood the basic purpose of each project. You might consider asking with more humility and curiosity next time.
I think the answer is rather simple and straight forward: just as there are many people who have created different GNU/Linux distributions because they thought changing a particular aspect would be an improvement, people want to improve security and offer their version.