Team Rocket has taken over the guardhouse on the east side of Route 43. They'll be happy to let you pass... for 1,000. If you don't want to pay, you'll have to loop around to the west. But that may be for the best; the grassy areas along Route 43 are the only places where you can catch a Girafarig in Gold and Silver, or a Farfetch'd in Crystal.
After you've crushed the Rockets, the guardhouse will fall back into friendly hands, and you'll be allowed to pass freely. And the new guard will thank you with TM 36 (Sludge Bomb), an awesome Poison-type attack.
#203 Girafarig
This palindromic giraffe is a solid Psychic Pokémon. It's half Normal which, on the plus side, means it can Stomp on opponents who would be less effected by Psychic (it also gives it full immunity to Ghost attacks). On the down side, it makes its Psychic attacks weaker than those of other Pokémon. But it gets great skills: Psybeam at level 41 and Dark's best skill (Crunch) at level 54.