gm_bigcity is based on New York City and twin towers are still standing, so Garry's Mod must take place sometime before 9/11.

Buildings based on World Financial Center already exist in Garryverse, and they were already built in 1987.

AIM-120 AMRAAM was invented in 1991, and was first operationally used in december 1992 and it already exist in Garryverse, however i don't see snow or any christmas decorations on maps.

So in conclusion, Garry's Mod takes place at least in 1993 since buildings based on World Financial Center, and AIM-120 AMRAAM already exist in Garryverse, i'm still trying to investigate clues when does Garry's Mod exactly take place