You finally come to the other end of town.  Stopping to take a quick break, you notice 3 buildings:

To your left you notice a large building.  There is a sign out in front of it that reads:
BATTLE SIMULATOR
NEED BATTLE EXP?  THEN THIS IS THE PLACE FOR YOU!
You decide to check the place out, since you think that the extra experience will help you prepare for your showdown with the gym leader.
Enter the building

To your right you notice a Pokemart where you can buy some supplies.
Enter the Pokemart

And finally, straight ahead you see Electro Gym.  The building that you've been looking for all this time.
Enter the Gym

Or, if you don't think that you're ready yet, you can go back.