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You have to finish something else before you can find her, don't remember what was it, but try to finish other quests or talk to Lusamine

she is in the building behind the trainer school

I was stuck in the same part, after wandering the map for a lot of time I've discovered that while the quest says that you need to speak to Gal Goodgirl, what you actually need to do is defeat Bea (gym leader ) in Dewford. Then go to Rustboro to talk to Gal.