Question: what do you do after you beat the veilstone gym leader in pokemon diamond?
Answer: Go to Route 214! After beating the Veilstone Gym, go to the northwestern corner of Veilstone City. You find Lucas/Dawn has got into trouble with two Team Galactic Grunts. Team up with Lucas/Dawn and do a Tag Battle against two Galactic Grunts to let Lucas/Dawn reclaim his/her Pokedex.