This is the tensioner for the main serpentine accessory drive belt. Gates was/is the GM supplier for the 12609719 Belt Tensioner and to AC Delco as P/N 38158. It fits all four years of the SSR, both 5.3 and 6.0 liter applications.
Part Number Cross-reference:
AC Delco 38158
Please note that your tensioner may come with either Gates or AC Delco packaging.