Guerrero was left off the list after the Court of Arbitration for Sport increased from six months to 14 months a ban for testing positive for cocaine contained in a tea.
Gareca, who has guided the South Americans to their first World Cup Finals since 1982, said the loss of the captain and top scorer was a blow but did not end their hopes.
"Paolo is a sensational player, an idol, but life goes on and we have to represent the country in the best way possible," he told reporters.
"We are going to be prepared for the maximum of demands and we are going to resolve all the possible inconveniences that present themselves.
"These boys grow when they come here, they are transformed when they pull on that shirt."
Guerrero left out 39-year old striker Claudio Pizarro but included forwards Andre Carrillo of Watford, Raul Ruidiaz of Mexican side Monarcas, and Jefferson Farfan, who plays for Locomotiv Moscow.
The latter two are small and quick but the side will continue to play the long-ball game that took them to Russia.
"We are taking two centre forwards with different characteristics," Gareca said. "Farfan can outjump any defence in the world. Raul Ruidiaz, with his stature, can win balls in the air. We have the means to hold up the ball in attack, as we've been doing."
Peru kick off their World Cup Group C campaign against Denmark on June 16 before facing France and Australia.
The Peruvian government stepped into the Guerrero debate on Wednesday by offering the player legal assistance.
President Martin Vizcarra called the ban "disproportionate" and said Peruvian officials would do all they could to support Guerrero's attempts to overturn the suspension in a Swiss federal court.
"It's unfair," Vizcarra said. "We are going to give support to the actions they are taking so the Foreign Minister (and) the ambassador to Switzerland can be available for Paolo and his lawyers."
(Reporting by Andrew Downie, editing by Ed Osmond)
