Gates was/is the GM supplier for the 12637202 Main Accessory Serpentine Belt and also supplies the part to AC Delco as P/N 6K923. It fits the first two years of the SSR, 2003 and 2004, 5.3 liter applications only.
Part Number Cross-reference:
AC Delco 6K923
Please note that your belt may come with either Gates or AC Delco packaging.