The Gold Region can be unlocked once you see an Archer Woman in the weapon shop in Mosso Village with a light bulb over her head. (Which for me, was fairly late in the game - but she might appear early on; I just didn't think to check until right before the Normal End.)
Anyway, in the Gold Region, all the monsters are made of gold. (And they drop a ton of gold, compared to other monsters.)
The Archer Woman who tells you about the Gold Region says you need to be about level 25 - but, like the metal region, that's only for the first screen.
You can access the second screen of the Gold Region at any time, but if your level is too low, the monsters there will annihilate you.
There's no real secret, or strategy, behind grinding in the Gold Region; just make sure you're a high enough level for the second screen, and you'll do fine.