Return to the Olivine Lighthouse (or go there now if you haven't gone already) and talk to Jasmine at the top of the Lighthouse. Remember that you can take the elevator if you've already talked to her! After giving her the Secretpotion, she'll give it to Amphy and it will cure Amphy's sickness, causing the Ampharos to brighten the room and the whole world! Well, maybe not the whole world, but it also brightened Jasmine's mood enough to cause her to return to her Gym, so heal up your Pokemon if you haven't done so already and get going to the Gym!
After leaving the Lighthouse, you'll get a call on your Pokegear from Baoba and he's letting you know that the Safari Zone is now open, so you might want to consider checking that out before your fight with Jasmine, too!
Since there's nothing else preventing you from taking on the Olivine Gym and since you just got your last Gym Badge, I'll cover Routes 47 and 48, which are accessible from the cave north of Cianwood City. They lead to the Safari Zone and are totally new to the game.
However, these two routes are totally optional and just take you to the Safari Zone. You can skip ahead to Olivine Gym if you would rather just get your Gym Badge. Remember, you can visit the Safari Zone and prior routes anytime you want now, so you can just flip back to the respective pages.