You're going to need clips to mount that thing... Better order them online, or pay $6/piece at the dealership.
You need a bag of each of these;
SET OF 15 NEW GM Rocker Panel Molding Clips 10431761 Free Shipping | eBay
15 Grand Prix Rocker Panel Moulding Clips 10323057 | eBay

I bought a few bags of each to do both on my car, ended up having a few left. I'll sell you what you need if you want, same price as Ebay, shipping is extra as always.

Good luck