Why does no one respect me?

I'm guessing a lot of this is in your mind. I highly doubt people are talking crap about you in the gym. People are focused on their own workout. That kind of stuff happens a lot in high school, but it sounds like you're not in high school anymore. It sounds like you have severe confidence issues, and it's true that people find it hard to respect someone who obviously doesn't respect himself. Figure out what you have to do to be happy with yourself first.