Gaps appear on garage doors between the door and just about anywhere else around the door. They can be caused by any number of things.
If you have a wood paneled door, sometimes the the panels can sag. In a case like this you will need to call a professional. Another very common reason for garage door gaps is caused when the slab at the bottom of the door sinks or moves. If the gap is not too big, you can pad it with weather stripping. If you have a gap that is, say , larger than three quarters of an inch, you can tack a piece of wood to close the distance.
If your gap is larger than three quarters of an inch, and we have seen some as large as 2 inches. In that case you will need to take some radical steps. You might try injecting concrete under the slab, but it will require a professional and it will be expensive.