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

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

Beat Koga — Fuchsia City Gym, badge 5 opens 11 areas. There are 42 trainers standing in them. The fight at this checkpoint is Koga.

Where to go

1. Sea Route 19

   Wild: Magikarp 100% · Lv 5, Tentacool 100% · Lv 5-40, Goldeen 50% · Lv 10, Poliwag 50% · Lv 10 +3 more

2. Sea Route 20

   Wild: Magikarp 100% · Lv 5, Tentacool 100% · Lv 5-40, Goldeen 50% · Lv 10, Poliwag 50% · Lv 10 +3 more

3. Sea Route 21

   Wild: Magikarp 100% · Lv 5, Tentacool 100% · Lv 5-40, Goldeen 50% · Lv 10, Poliwag 50% · Lv 10 +8 more

4. Kanto Power Plant

   Wild: Magnemite 30% · Lv 21-23, Voltorb 30% · Lv 21-23, Pikachu 25% · Lv 20-24, Magneton 10% · Lv 32-35 +4 more

5. Seafoam Islands

   Wild: Magikarp 100% · Lv 5, Goldeen 50% · Lv 10, Poliwag 50% · Lv 10, Horsea 35% · Lv 31-33 +14 more

6. Pokémon Mansion

   Wild: Grimer 50% · Lv 31-33, Koffing 50% · Lv 31-33, Ponyta 40% · Lv 28-34, Growlithe 20% · Lv 32 +4 more

   Trainers: Burglar Lv 34, Burglar #2 Lv 38, Burglar #3 Lv 34, Scientist Lv 29, Scientist #2 Lv 33, Scientist #3 Lv 34

   Pick up: TM14 Blizzard, TM22 Solar Beam

7. Cinnabar Island

   Wild: Magikarp 100% · Lv 5, Goldeen 50% · Lv 10, Poliwag 50% · Lv 10, Horsea 25% · Lv 15 +6 more

   Pick up: TM38 Fire Blast

8. Route 19

   Trainers: Beauty Lv 27, Beauty #2 Lv 30, Beauty #3 Lv 29, Swimmer Lv 30, Swimmer #2 Lv 29, Swimmer #3 Lv 30 +4 more

9. Route 20

   Trainers: Beauty Lv 35, Beauty #2 Lv 30, Beauty #3 Lv 31, Beauty #4 Lv 30, Bird Keeper Lv 30, Jr Trainer F Lv 31 +4 more

10. Route 21

    Trainers: Cue Ball Lv 31, Fisher Lv 28, Fisher #2 Lv 31, Fisher #3 Lv 27, Fisher #4 Lv 33, Swimmer Lv 33 +3 more

11. Fuchsia City Gym

    Trainers: ★ Koga Lv 43, Juggler Lv 31, Juggler #2 Lv 34, Juggler #3 Lv 38, Juggler #4 Lv 34, Tamer Lv 34 +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		Tackle Smog Sludge Smokescreen	HP40Attack65Defense95Sp. Atk60Sp. Def45Speed35Total340
	Muk Lv 39 Poison		Disable Poison Gas Minimize Sludge	HP105Attack105Defense75Sp. Atk65Sp. Def100Speed50Total500
	Koffing Lv 37 Poison		Tackle Smog Sludge Smokescreen	HP40Attack65Defense95Sp. Atk60Sp. Def45Speed35Total340
	Weezing Lv 43 Poison		Smog Sludge Smokescreen Selfdestruct	HP65Attack90Defense120Sp. Atk85Sp. Def70Speed60Total490

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

It also doesn't say when Kanto Underground Path opens up, so it is in no part above; it has its own route page.