I'm not quite following what you are asking.
Do you mean that you have a GMC that you want to put a billet grill in?" or are you wanting to convert a Chevy to GMC style hood?
I've got a notion that you are going to be on your own making one. If it were me I'd do a mockup using something like Masonite or MFD that was closest to the desired thickness that I got at Lowes or Home Depot or my favorite local lumber yard so I had the exact shape of the bars as I wanted them and take the pattern to a shop that can waterjet it out of aluminum. You can throw a way a lot of mistakes from a 15.00 4x8 sheet of that stuff and not feel bad.
One thing to remember is that those (and Chevy) grill openings taper from bottom to top slightly and the bars will have to be graduated in length accordingly to be right.
The other way is a lot of cutting and fitting with aluminum flat bar. Or get square aluminum bar stock and cut and shape the bars. This chart gives the ability to bend of different alloys.
https://www.clintonaluminum.com/whic...oy-bends-best/
That would give you the alloy to look for which probably would be 3003 or 5052 as a lot of the others don't bend well. I'm not finding either in square stock though. Most of the square stock is too hard for bending well.