Gates was/is the GM supplier for both Belt Tensioners and the Idler Pulley for all four years of the SSR, both 5.3 and 6.0 liter applications. You can see traces of that connection when comparing the part numbering between AC Delco and Gates.
AC Tensioner 12580196 38159 38159
Main Tensioner 12609719 38158 38158
Idler Pulley 12669569 36299 36299
Please note that your part may come with either Gates or AC Delco packaging.