France led 2-0 thanks to Giroud's 30th goal for his country and a fine counter-attack rounded off by Monaco star Thomas Lemar but Luis Muriel and Radamel Falcao got Colombia back on level terms before Juan Quintero's late penalty completed a stunning comeback in Paris.
Les Bleus are considered among the favourites to win the World Cup but Giroud accepts France must show a response on Tuesday against tournament hosts Russia.
"A football game is always two halves," Chelsea striker Giroud told reporters. "We can't become relaxed and too confident.
"Especially on the important things we have during a football game of such high intensity: engagement, aggressiveness, determination. Those are essential things to succeed.
"I hope this slap during the second half will be a warning to us to bounce - and to do that before Tuesday."
Juventus midfielder Blaise Matuidi says France should not panic despite losing from such a strong position.
"No, it is not worrying," Matuidi said. "Of course, we have to be conscious that we did some good and bad things.
"Those bad things cost us the victory and made us lose. It is our duty to notice what was wrong and to try to correct it on Tuesday.
"But I don't want to say it is worrying. We don't have to go that far, not everything has to be thrown away."
Giroud's Chelsea team-mate N'Golo Kante highlighted individual errors as the reason for France's defeat, as well as tactical confusion after Colombia drew level.
"In the second half, Colombia reacted really well, they had a big pressing game that put us in trouble," Kante said. "Then, we had problems developing our own way of playing.
"At 2-2, we didn't even know what we should do - attack or defend. They won 3-2 but I think we lost that game because of ourselves, due to our own mistakes."