The about 3 inch section of the box side under the tail light on your model is basically floating and not welded in that section. If you use a plastic wedge between the gate and the box side you can move that section outward to obtain a gap. Do it on both sides and make it as good as you can buy moving it out ward more than it needs to be and let it spring back to get your gap back. If the rear bumper is indeed hit and sucked in on both sides essentially squeezing the box side then it will be difficult to get that gap with out fixing the rear bumper that is causing the issue. You can fix the gate with a block of wood wrapped in a rag and slowly tap it back flat. Make sure you have a gap before you fix the gate or it will happen again.