I've had this problem since I bought my GX. I'm guessing the original owner bumped into something or maybe it's a defect? I used to be able to push it in and it would stay for awhile but eventually the plastic tabs wore out. I pulled down the plastic in the wheel well to see what was there (nothing).
I couldn't find any resolution from inside as there is nothing relating to this area of the bumper. Only the two screws in the fender well and the plastic tabs.
This is what it eventually came to look like.
I pulled the plastic bumper back and scuffed it up with a grinder.
This is what I used to bond it.
high-tech pressure application LOL
This is what it looks like three weeks later. A little bit of a wider gap compared to the other side but I can live with it.A big improvement from what I HAVE been living with.