Guerrero this week had his six-month suspension for doping extended to 14 months by the Court of Arbitration for Sport (CAS), ruling him out of football's flagship tournament in Russia, which begins on June 14, reports Xinhua news agency.
"We believe the penalty is totally unfair," Gareca told a press conference here. "We believe that FIFPro needs to take a position. This needs to reverberate globally. It could happen to any other star, any other player.
"We have been left without our captain. We are really upset at how this has been handled. Today Paolo should be here training with us. You have to add the sanction to the isolation that the player is subjected to. He is sidelined from his teammates, training... He is sidelined from society. It's something that needs to be revised urgently."
The penalty, backdated to November, was halved by FIFA's appeal committee less than two weeks later.
Guerrero, who is Peru's all-time leading scorer with 32 goals from 83 matches, has maintained his innocence and says the substance was unwittingly consumed in contaminated tea.
The case was referred to the CAS after a challenge by the World Anti-Doping Agency. The Flamengo striker also appealed in a bid to clear his name.
A CAS panel said it accepted the 34-year-old's claim that he did not attempt to enhance his performance by taking a prohibited substance.
But it considered that the former Bayern Munich player bore "some fault or negligence".
"We know of Paolo's innocence, we know he is a sensational player, with great professionalism and a well-deserved idol," Gareca said.
"But our story continues. We must represent the country in the best way possible and we believe in the guys who are available."
(This story has not been edited by Business Standard staff and is auto-generated from a syndicated feed.)