What does 18KT HGF inside a ring mean?

The GF stands for gold-filled, I believe the H stands for hard.
The item is gold-filled, meaning it has a micro-thin layer of 18K gold over a base metal.
Older gold-filled items such as pocket watches used to be stamped 1/20 12K GF, meaning 12 Kt gold (50% pure) is layered over the base metal, with the gold weight being at least 1/20th of the total weight of the item.