The Arizona Democrat, who was shot in the head during a 2011 attack that killed six people, said too many children were dying and something must be done.
"Violence is a big problem. Too many children are dying. We must do something," Ms Giffords told the senate judiciary committee hearing.
"It will be hard, but the time is now.
"You must act. Be bold, be courageous. Americans are counting on you."
Escorted into the room by her husband, former NASA astronaut Mark Kelly, Ms Giffords was a surprise witness at the year's first congressional hearing on the causes of and possible solutions for gun violence.
The hearing comes six weeks after a gunman stormed into a school in Newtown, Connecticut and killed 20 small children and six adults.
Ms Giffords came face to face at the hearing with leaders of the National Rifle Association, who reiterated their call for arming security forces at schools.
Wayne LaPierre, the NRA's executive vice-president and the gun lobby's leading voice, said gun control measures had failed in the past.
"Proposing more gun control laws - while failing to enforce the thousands we already have - is not a serious solution to reducing crime," Mr LaPierre said in prepared testimony.
"Law-abiding gun owners will not accept blame for the acts of violent or deranged criminals. Nor do we believe the government should dictate what we can lawfully own and use to protect our families."
Mr LaPierre told the panel that the NRA opposed universal background checks for gun owners, a reversal of an earlier position, saying the current system had failed because the administration did not aggressively prosecute violators.
"When it comes to background checks, let's be honest - background checks will never be universal because criminals will never submit to them," he said.
Mr LaPierre argued with Senator Dick Durbin on this point. The Democrat said criminals would not go to buy guns because they would face a background check.
The Newtown massacre has prompted a spirited national debate about gun rights, and led president Barack Obama to propose mandatory background checks for all gun purchases and reinstatement of an assault weapons ban.