I don’t have much experience with Ground Force. Maybe contact Ground Force and let them know the kit/part # you have and see what they say about it being able to be used on your ‘15 Suburban LTZ?
I know some other companies like Belltech do use the same lowering springs, shock extenders, and swaybar links for to lower the rear for ‘07-‘20 MY SUV’s.
Are you planning to lower the front also? I know there are different part numbers for front drop spindles used for ‘07-‘14 MY vs ‘15-‘20 MY.