Okay. Get her to go to YOUR gym with YOU. Don't leave her in the dust while you're there, do things
with her. Make it a workout for the both of you.
I've dealt with the same problem you're dealing with now, except from the female end. Kinda dealing with it right now.
First of all, compliment her. Be sincere, and don't say anything you don't mean. Let her know you love her, and not just for sex.
When I started working out again, my sex drive went up. After my last breakup it kinda dropped for a while (there are gonna be people who don't believe that's possible
). So that might be another thing to consider. She'll have more energy and feel better about herself.
Mention it when you notice a difference. Don't let her think that exercise isn't producing results. Instead of going out for dinner, offer to cook, and cook something healthy.
There's lots of ways you can help her without even actually dragging her to the gym.