You can re-challenge the gym leaders in Pokemon Soul Silver by finding them when they're away from the gym, and then calling them at a certain time. Here's a list of how to re-challenge each gym leader:
- Falkner - Go to the Department Store in Kanto during the daytime on a Monday to get Falkner's number. Call him on Saturday morning for a rematch.
- Bugsy - Go to the Viridian Forest on a Thursday daytime to find Bugsy. Call him on Thrusday afternoon to arrange a rematch.
- Whitney - You can find Whitney in the Goldenrod Department Store on any afternoon. Get her number and then call her on Saturday afternoon to arrange the rematch.
- Morty - Find Morty at the Bell Tower on a Monday daytime. You can then call him on a Tuesday night to arrange for a rematch.
- Chuck - You can get Chuck's phone number from his wife, who stands outside of his gym. You can then call him on a Wednesday night to arrange the rematch.
- Jasmine - You can find Jasmine at the Olivine Dining Room. Once you've got her number, call her on a Wednesday afternoon to re-challenge her.
- Pryce - You'll find price at the Lake of Rage. Surf to the west to locate him. You can then call him on a Monday morning to arrange the rematch with him.
- Clair - You can find Clair most mornings down by the water at the Dragon's Den. Call her on a Friday night to arrange a rematch with her.
To find Kanto gym leader's whereabouts, you can have a look at Serebii.net. Remember that the rematches will be much more difficult than the original gym leader fights!
Here's an awesome video showing the gym leader rematches in full: