The Golden Country is an allusion to the Garden of Eden from the Bible.
Winston dreams of the Golden Country, which is a very Eden-like place where he sees a “girl with dark hair” (Ch. 3), who alludes to Even.
It was an old, rabbit-bitten pasture, with a foot-track wandering across it and a molehill here and there. … Somewhere near at hand, though out of sight, there was a clear, slow-moving stream where dace were swimming in the pools under the willow trees. (Ch. 3)
This idealized scene, and the woman reaching out to him, make Winston Adam and the girl with the black hair Eve. Winston has created a fantasy for himself of Biblical proportions, and Orwell is reminding us of Winston’s role in his society, especially later when Julia and Winston meet in a place that is very much like the Golden Country (Ch. 10).
Authors use allusions to place their characters in broader cultural themes. Most readers would recognize the connection between the Garden of Eden and the Golden Country, and the innocence and sin of both couples. The connection allows us to make a deeper meaning from a simple description that actually carries much more weight through the allusion.