The Golden Rule operates at the small scale of an iterated prisoner's dilemma where two parties are directly interacting.

Kant's wording scales up to include the emergent properties of a society which operated according to that rule.

The Golden Rule doesn't cover cases like, "Should I throw this trash in a lake." because there's no concrete person being "done unto". Kant's rule does: Would you want to live in a world where everyone threw trash in the lake? Probably not.