The Fuchsia Gym looks incredibly simple at first glance — the Gym Leader is right in front of you! But then as you take a few steps forward, you'll hit an invisible wall (that will appear in front of you and make a sound) and you'll also see the Gym Leader all over the place.
Yeah, all of the Trainers in the Gym look like the Gym Leader, Janine, until you talk to them. Once you talk to the Trainers, they'll reveal their true form.
I'll take away the most difficult part of the Gym for you: the real Janine is the one directly in front of you as soon as you enter the Gym. She'll line up perfectly between those two statues near the entrance. Any of the other "Janines" are actually just fakes and you'll have to battle some Trainers when you talk to them.
Because of knowing this, thanks to me, of course, you don't have to fight a single Trainer in the Gym. You can just hug the right wall, then hug the back wall, then sort of fidget around until you get to the center of the room and fight Janine. But... why pass up on the EXP you can earn from those Trainers?
Fight as many or as little as you'd like, then make sure your Pokemon are well healed before taking on the Gym Leader, Janine — Koga's daughter, by the way.
Any good Steel-type Pokemon is going to completely wreck Janine's team, rendering her Poison Jab and Sludge Bomb attacks totally useless and halving the damage of all of her other attacks. All of them. But if you don't have a Steel-type Pokemon in your arsenal, you shouldn't have too much to worry about, either — her Pokemon aren't too threatening.
Also, despite her entire team being Poison-type, none of her Pokemon are weak to Ground-type attacks thanks to their typing or Weezing's Levitate ability, so try use Psychic- or Fire-type attacks against her instead.
She starts off with her Crobat, which will likely try confuse your active Pokemon with either Confuse Ray or Supersonic. Supersonic has a chance of missing where Confuse Ray does not, so hope that it uses Supersonic and misses! It will also Screech to lower your Defense by a lot and soften your Pokemon up for the next Pokemon she sends out. Remember to switch out your Pokemon if it gets hit with Screech or becomes confused, because it's better to avoid taking chances!
Weezing is at a rather low level, only at level 44, but it can still debilitate you with Toxic and can use Explosion to take itself out and inflict massive (possibly lethal) damage to whatever its facing. Be careful to take it out quickly to avoid triggering Explosion! Also, Weezing has Levitate, so Ground-type moves are ineffective against it.
Her two Ariados will also try mess you up, but their attacks are pretty lackluster. Each possess Poison Jab and Pin Missile, but their other moves are mostly for show. Swagger can help you out if you follow up with a berry or something to snap your Pokemon out of confusion, giving your Pokemon some extra Attack.
Venomoth is her last Pokemon. It has a Sitrus Berry and a variety of Special-based attacks in its arsenal, as well as Double Team to make its evasion increase. Watch out and make sure that it doesn't get too many Double Teams on you! Fire-, Flying-, or Psychic-type attacks are your best bet to take care of it. Signal Beam and Sludge Bomb can hurt and will hurt more than her other Pokemon's attacks (except Weezing's Explosion), but they will be a piece of cake compared to what Erika could do to you.
After beating Janine, she'll relinquish the Soul Badge as well as TM84, which contains Poison Jab.
Now, Route 19 to the south is blocked, so you won't be able to head that way just yet. You can head east of Fuchsia City onto Routes 12 - 15 and eventually over to Lavender Town, but you can get there even easier by flying to Saffron City and walking east.
You aren't missing anything critical if you take the short route, but I'll go ahead and cover the long way in case you want to take that path.