Completely agree with @gardenstategal here. Your mom is wrong about this.
Mock Trial is something pretty common and doesn’t require nearly as much commitment as gymnastics.
Since you have a passion for gymnastics and it requires a lot of focus and commitment, it will take you far.
You can always do Mock Trial in college if you really want to but your window to do competitive gymnasitcs is small and I would take advantage of that.
If you step away from gymnastics and try to get back into it, it’s a much steeper hill to climb than re-engaging in Mock Trial.