Gengar locations — where to catch

In Lumiose City (Legends: Z-A)

• Centrico Plaza — Static Encounter
• Wild Zone 20 — Walk

In Paldea (Scarlet & Violet)

• Levincia — Trade

In Kitakami (Scarlet & Violet – The Teal Mask)

• Mossfell Confluence — Walk

In Hisui (Legends: Arceus)

• Space-time distortions — Walk

In Sinnoh (Diamond, Pearl & Platinum)

• Old Chateau — Walk

Other games (3)

• Sword / Shield (Gen 8): Giant's Cap, Giant's Mirror, South Lake Miloch, Stony Wilderness
• Ultra Sun / Ultra Moon (Gen 7): Thrifty Megamart
• Sun / Moon (Gen 7): Thrifty Megamart

Gengar — full Pokédex entry