There is no Vs Seeker in HG/SS. Instead NPCs (in game characters) give you their phone number that registers in the PokeGear. They will occasionally phone you up for a rematch or you can ring them (not quite sure). Gym leaders are only available for rematches on certain days visit this page for more info:
http://www.serebii.net/heartgoldsoulsilver/gym-rematch.shtml
