Part 8: Beat Koga — Fuchsia City Gym, badge 5

Surf (HM03, Safari Zone — Secret House), the Good Rod (Fuchsia City — House 2), the Super Rod (Route 12 — Fishing House)

Beat Koga — Fuchsia City Gym, badge 5 opens 11 areas. There are 44 trainers standing in them. The fight at this checkpoint is Koga.

Where to go

1. Pokémon Mansion

   Wild: Grimer 30% · Lv 28-30, Koffing 30% · Lv 28-30, Raticate 30% · Lv 32-36, Growlithe 15% · Lv 30-32 +5 more

   Trainers: Burglar Arnie Lv 34, Burglar Lewis Lv 34, Burglar Simon Lv 38, Scientist Braydon Lv 33, Scientist Ivan Lv 34, Scientist Ted Lv 29 +1 more

   Pick up: TM14 Blizzard, TM22 Solar Beam

2. Seafoam Islands

   Wild: Magikarp 100% · Lv 5, Horsea 80% · Lv 15-30, Krabby 80% · Lv 15-30, Seel 60% · Lv 25-35 +9 more

3. Kanto Power Plant

   Wild: Magnemite 30% · Lv 22-25, Voltorb 30% · Lv 22-25, Pikachu 25% · Lv 22-26, Magneton 15% · Lv 31-34 +3 more

4. Sea Route 19

   Wild: Magikarp 100% · Lv 5, Tentacool 100% · Lv 5-40, Horsea 80% · Lv 15-25, Krabby 80% · Lv 15-25 +5 more

5. Sea Route 20

   Wild: Magikarp 100% · Lv 5, Tentacool 100% · Lv 5-40, Horsea 80% · Lv 15-25, Krabby 80% · Lv 15-25 +5 more

6. Sea Route 21

   Wild: Magikarp 100% · Lv 5, Tangela 100% · Lv 17-28, Tentacool 100% · Lv 5-40, Horsea 80% · Lv 15-25 +6 more

7. Cinnabar Island

   Wild: Magikarp 100% · Lv 5, Tentacool 100% · Lv 5-40, Horsea 60% · Lv 5-15, Krabby 60% · Lv 5-15 +11 more

   Pick up: TM38 Fire Blast

8. Route 19

   Trainers: Sis And Bro Lia & Luc Lv 30, Swimmer♀ Alice Lv 30, Swimmer♀ Anya Lv 27, Swimmer♀ Connie Lv 29, Swimmer♂ Axle Lv 27, Swimmer♂ David Lv 29 +5 more

9. Route 20

   Trainers: Bird Keeper Roger Lv 30, Picnicker Irene Lv 30, Picnicker Missy Lv 31, Swimmer♀ Melissa Lv 31, Swimmer♀ Nora Lv 30, Swimmer♀ Shirley Lv 30 +4 more

10. Route 21

    Trainers: Fisherman Claude Lv 31, Fisherman Nolan Lv 33, Fisherman Ronald Lv 28, Fisherman Wade Lv 27, Sis And Bro Lil & Ian Lv 33, Swimmer♂ Jack Lv 37 +3 more

11. Fuchsia City Gym

    Trainers: ★ Koga Lv 43, Juggler Kayden Lv 38, Juggler Kirk Lv 31, Juggler Nate Lv 34, Juggler Shawn Lv 34, Tamer Edgar Lv 33 +1 more

★ Koga — Fuchsia City Gym

4 Pokémon · level cap Lv 43 (every cap) · Koga’s own page

	Pokémon	Ability & item	Moves	Base stats
	Koffing Lv 37 Poison		Selfdestruct Sludge Smokescreen Toxic	HP40Attack65Defense95Sp. Atk60Sp. Def45Speed35Total340
	Muk Lv 39 Poison		Minimize Sludge Acid Armor Toxic	HP105Attack105Defense75Sp. Atk65Sp. Def100Speed50Total500
	Koffing Lv 37 Poison		Selfdestruct Sludge Smokescreen Toxic	HP40Attack65Defense95Sp. Atk60Sp. Def45Speed35Total340
	Weezing Lv 43 Poison		Tackle Sludge Smokescreen Toxic	HP65Attack90Defense120Sp. Atk85Sp. Def70Speed60Total490

Everything you can have caught by this checkpoint · Open the interactive guide

It also doesn't say when 4 of its areas open up, so they are listed together in the last part rather than guessed into the story.