The militants used a truck and armored vehicle stolen from the security forces to carry out the coordinated attack.
The deputy Afghan minister of interior for security, Gen. Murad Ali Murad confirming the death toll said 48 security force members and 110 civilians were wounded in the twin blasts.
"21 police officers from Gardez were martyred and 48 others were wounded. Meanwhile, 20 Gardez residents were martyred and 110 others were wounded in the attack," the Tolo News quoted Murad as saying.
The police chief for Paktia province Toryalai Abdyani, was among those killed in the deadly suicide bombing.
Meanwhile, Afghan Chief Executive Abdullah Abdullah sent his condolences to the victims' families, saying that government will investigate the incident.
He said more than 4,000 kilograms of explosives and many military vehicles were used in the biggest attack launched against security forces so far.
According to media reports, at least two attackers have also been killed by the security forces.
Insurgents detonated a car bomb and then entered the training center, the MoI said.
The Taliban has claimed responsibility for the attack. (ANI)