Pokémon Gaming CentralFor topics that aren't necessarily restricted to one game, Pokémon Gaming Central ranges from comparing and contrasting the differences in the gaming generations to discussing the gaming franchise as a whole.
You're not new to the site though, so you should know about the existing help threads by now. :/
Anyway, you need to go back to the Game Corner and defeat the Rocket members there. You push a switch on the wall behind a poster after fighting him and you go in there and defeat that hideout. Then you'll get an item to show the ghosts in Pokemon Tower. You'll complete Pokemon Tower and then get the PokeFlute to move Snorlax that are in your way. And wake sleeping Pokemon up in battle, I think. Then you should be able to access that gym there. But I recommend defeating Silph Co. first, which is in Saffron. The big building with the crazy sounding music and the warp panels all over. You'll get a Master Ball for your efforts and yes, it is required to complete. I think you might actually have to do that before you can fight Sabrina. Good luck.
If you misunderstood anything or have more questions about LG, please use the Q&A thread in Advance Generation Gaming. :)