In a relationship, it is really important for two people to have the same interests. But for a girl who doesn't like sports, she really doesn't have much motivation to exercise. Even if she is willing to accompany you, it won't last long.
Losing weight is the biggest motivation for girls' fitness, which can make girls exert unusual energy. You want to persuade your girlfriend to take part in physical exercise. I want to say that she is fat is the best way.
Although you two may quarrel, you may be beaten by your girlfriend. But then your girlfriend will definitely take part in physical exercise more actively than you. Every girl wants her figure to be perfect. She may not be self-disciplined, but if someone takes her, she will definitely go.
The method is as simple as that, let's see if you dare. You don't need to think too complicated at all. This is not a matter of compromise or concession. Just losing weight.
When losing weight becomes a habit, she will go without your advice. In fact, the time you need to convince is very short, just a few days ago. Since you say she is fat and she just goes to exercise, we must pay attention to the praise effect in our daily life.
Every day your girlfriend exercises, you should say that she has lost weight. Even if she doesn't, you should choose those places that are not obvious enough to say that they are effective. I think this effect is the motivation for girls to continue walking the next day.
After half a month or even a month, you two went to the gym together and ignored you at all. Your girlfriend will take the initiative to participate in sports, and the habit is actually so easy to form.
There are many ways to solve problems in life, some of which are very effective, but also very dangerous. I'm very optimistic about your idea, but it's up to you to use the method I provide.
I hope you can go to the gym together in the future. Having a good body is really more important than anything else.