From the Super Mario Wiki
The Gold Ghost is the first ghost Luigi encounters in Luigi's Mansion. It is first met in the Parlor, which is accessed after obtaining the key from a Gold Ghost in the Foyer, after blowing all six candles out with the Poltergust 3000. The paintings in the room would come to life, then the doors would seal up and three of these ghouls would appear, one-by-one. These normal ghosts would only have ten HP and would be easy to beat once Luigi was accustomed to his new vacuum. This ghost's tactic of choice is to move directly towards Luigi and attack using a short-range uppercut. They would reappear countless times in the mansion, found in groups of two or more and could be hurt by any element. Professor E. Gadd also trains Luigi with these kinds of ghosts in the training room. They chase Luigi but don't attack him, unlike the ones found in the mansion.
A Gold Ghost cameos in several later Mario games. In Mario & Luigi: Superstar Saga, a Gold Ghost briefly cameos in Starbeans Cafe.
In Mario Power Tennis, the Gold Ghost, as well as several other generic ghost species from Luigi's Mansion, appear on the Luigi's Mansion tennis court and in the minigame Terror Tennis. In Mario Super Sluggers Gold Ghosts appear in the unlockable stadium Luigi's Mansion. In Mario Party 8, Gold Ghosts cameo in the minigame Specter Inspector. The Gold Ghost also appears as a part of a trophy in Super Smash Bros. Brawl.
Gold Ghost make an appearance in Mario Sports Mix as spectators in the sporting events.
In the Nintendo Land minigame Luigi's Ghost Mansion, the Mii attacking the other four Mii players use a Gold Ghost costume.
- Greenies from Luigi's Mansion: Dark Moon are similar to Gold Ghosts because they are both the first ghost encountered in their respective appearances, they both have 10 HP and take away 20 HP if they successfully attack Luigi and they both have a counterpart ghost which yields a lot of treasure.