the description on the kit you have says it has a pocket that goes under the CD player and i think the CD player sits flush. some GM vehicles are quite shallow and that is why other kits had the extensions.
if your car needs extensions, then you need a different kit that comes with them. if your used DC player didn't come with it's trim ring - then yes, you need to buy a trim ring. either the Kenwood or a black plastic one to fit the kit.
some of the Metra kits can flush up the HU and it looks great - but requires extra depth in the dash. most of the kits expect you to have the trim ring for the HU.
It doesn't mention needing the trim ring. if you can't return the kit (they aren't that expensive) then decide if you want to try it and see what you can make work, or if you want to get a different kit. maybe a local shop can hook you up with a trim ring - i always keep the extra parts from the dash kits, i have a selection of random pieces that have come in handy over the years.