I have another GM vehicle and while it is not cross bars, it gives you tie downs.
Purchased stainless "U" bolts at the home improvement store. They come with nuts and a strip of metal between the 2 legs. I think the size I used was 1/4 inch. The size is determined by the slot in your roof rails. I made a second cross strip and purchased additional stainless nuts.
I loaded the assembly in this order... nut, cross strip, second cross strip, and nut.
the last nut is positioned at the end of the "U" bolt.
Remove the end of the roof rail and slide the "U" bolt assembly into the slot.
you want one cross bar on the inside and the other on the outside.
Slide the "U" bolt to the desired location and tighten the nuts on the outside and that locks
the "U" bolt in place. This does not give you a cross bar but now you have tie downs