How can I talk to a girl at the gym?

First off, why would you want to make your ex jealous? If it's over, it's over. You're too old to play the silly little make the other person jealous kind of thing. If it was a bad breakup, then you don't want anything to do with her anyway, and who cares what she thinks. If you two are still friends, then you shouldn't want to make her feel jealous. Be a real man, be happy for her, and move on.

As far as talking to a girl at the gym, I know what you mean. You almost feel like you have to have a good reason to go talk to her. And if she's good looking, she probably has a ton of guys talking to her, and you think she gets tired of it. But If you see her a lot, maybe the first time, as soon as you spot her, go over, be confident, look her in the eyes, and say, "I just wanted you to know that you have very beautiful ______. " Be specific. If she has beautiful eyes, or gorgeous hair, then say that. Obviously don't say boobs or butt, because that's not the way to go. But tell her that she has very pretty eyes. After she says thank you, say "no problem. " And then walk away. This way, you're not some guy drooling all over her like every other guy, trying to impress her. The fact that you gave her a compliment and then walked away makes you seem different then other guys. You don't need some witty or clever statement or pickup line.

But then that opens the door for next time you see her. Next time you see her at the gym, you can go up to her and say, "hey, you're that girl with the beautiful eyes. " And there's your conversation starter right there.