There used to be GS-u stores in Poland in 80's. They sold the best trousers in the whole world. therefore "spodnie z gs-u" became a slang expression for trousers that all trousers in the whole world pale in comparison to
kurwa1: check out those magenta spodnie on Wojciech Wojda
kurwa2: man they're liek totally spodnie z gs-u!