By leveling up:
* Level 1: Absorb, Poison Sting
* Level 4: Vine Whip
* Level 7: Growth
* Level 10: Razor Leaf
* Level 13: Acid
* Level 16: Sleep Powder
* Level 19: Stun Spore
* Level 22: Petal Dance
* Level 25: Sweet Scent
* Level 28: Solarbeam
* Level 31: Toxic
* Level 34: Hidden Power (type varies)
* Level 37: Shadow Ball
* Level 40: Sludge Bomb
* Level 43: Aromatherapy
* Level 46: Sunny Day
* Level 49: Giga Drain
* Level 52: Energy Ball
By TM:
* TM06: Toxic
* TM10: Hidden Power
* TM11: Sunny Day
* TM16: Light Screen
* TM17: Protect
* TM21: Frustration
* TM27: Return
* TM29: Psychic
* TM32: Double Team
* TM33: Reflect
* TM34: Substitute
* TM42: Facade
* TM44: Rest
* TM45: Attract
* TM48: Skill Swap
* TM50: Overheat
* TM58: Sky Attack
* TM64: Explosion
* TM87: Swagger
* TM90: Substitute
* TM94: Aeroblast
By breeding:
* From Oddish: Chlorophyll
* From Vileplume: Effect Spore
By Move Tutor:
* From Move Tutor: Synthesis, Sludge Bomb
Note: Some moves may not be available until after you have obtained a certain item, like the Hidden Machine (HM) for Cut.
Remember that Gloom can also learn a variety of moves through other means, such as breeding or Move Tutors. Keep an eye out for these options as you progress through the game!