I have 4 68's and one is a GT/CS.
I have some tubulars I used on the GTCS and a GT grille I did up, and was just tinkering with some spare grilles, stock and GT I have, and noticed the difference and could find no specific and definitive reference.
Did the GT/CS grille a while ago and researching the color was "fun". Ended up with a Ford "Dark Titanium Metallic" I could get here in a rattle can, that was close enough for me. Wasn't worth the $100+ for paint an then finding a painter to spray it to some of the codes people there posted as correct. (and don't have the equipment and more than enough projects to get to without adding in learning to paint... maybe later...) The info I saw there was the grille was the dark grey and the mounting tabs were black. I did the Marchal brackets the same as the grille, which I am unsure is correct or not. I didn't have one to compare, an after market stock '68 or service replacement was on the car when I got it. I saw similar info on using the 67 grille on a GTCS, and modified a repro to be "close" as OEM GTCS ones seem very difficult to find.
There was long thread on the GTCS forum on many of the paint variants available. And quite a few on the grille restoration.
Thanks for the replies.