TUCSON — Tuesday was not just a day for Tucson to remember the victims of the deadly shooting that severely injured Representative Gabrielle Giffords. It was also a day when residents could see firsthand the nation’s gun debate play out in a busy parking lot outside a city police station.

On one side was a councilman, who supports gun control, leading an effort to give $50 grocery store gift cards to anyone who turned in their guns to police. And on the other was an event organized by a state senator that turned into an open, unregulated, and legal marketplace for firearms.