Send five Postcards from GO to SV to receive a Golden Lure Module. While this is active, a golden PokeStop will yield 5, 10, 15, 20, or 25 Gimmighoul Coins when it's spun. Further, golden PokeStops have a chance to spawn Gimmighoul, which have a chance to drop Gimmighoul Coins when caught.
It'll still take you a while to rack up 999 coins, but using Golden Lure Modules is the most effective way to go about it. Gimmighoul also occasionally drops a coin when registered as your Buddy Pokemon, but this is too infrequent to be a viable main method.
Gimmighoul Coin (Bulbapedia)
