Where to catch Gloom

In Blueberry Academy (Scarlet & Violet – The Indigo Disk)

• Coastal Biome — Walk

In Hoenn (Ruby, Sapphire & Emerald)

• Route 121 — Walk
• Route 123 — Walk
• Safari Zone — Walk

In Johto & Kanto (Gold, Silver & Crystal)

• Route 24 — Walk
• Route 5 — Walk

In Kanto (Red, Blue & Yellow)

• Cerulean Cave — Walk
• Route 12 — Walk
• Route 13 — Walk
• Route 14 — Walk
• Route 15 — Walk

Gloom — full Pokédex entry