How do I fix this? Is it the spring or something else?

Looks like the concrete falls off on the right.

The gasket at the bottom of your overhead compresses to help create a seal along an uneven surface, but it can only do so much. Your gap may be too large to seal before the door bottoms out completely on the left. That said, there is adjustments that set where the door comes to a stop when closing, check your manual and see if the overhead can lower further.

If you check the level of both the door and the slab it will let you know which one is causing the gap. I would guess that a gasket would be the best fix. If it is the door DO NOT try to fix it yourself. Those springs can very easily kill you. I have known life long contractors that will do everything in a house but a garage door. Garage doors are not DIY.

There isn't enough in the picture to be 100% certain. In no particular order, these are the three things that I would check first:

1. The cable could be too short on that side
2. The track could be slightly too high on that side
3. The floor is angled and has a natural gap

I'm not at all experienced in fixing these but some ideas come to mind: I doubt it's the spring, that would just control rotation of the main rod unless it somehow bent the rod. Could be the tracks on the side are becoming misaligned. That could be from loose track. That might be hard to tell with the door installed due to weight, someone might know of a way to check this.

You also might need to consider that the floor underneath is sinking on the gap side. Are there any cracks in the garage floor? Any cracks on nearby walls? This can happen naturally or due to a leaking pipe where water underground washes away the supporting sand/dirt under the building.