I'm no expert, but a basic response to your questions can be had.

Accounts agree that there was no physical torture; but interrogations were long and harsh(11+ hrs/day for 61 days leading up to his show trial) and included death threats. He was in solitary confinement in the Lubyanka and the Soviets controlled the flow of information to him.

His son, investigated the issue.

“He was not tortured,” says Powers Jr., founder and chairman emeritus of the Cold War Museum in Warrenton, Virginia. “But there were bright spotlights, grueling questions, sleep deprivation, threats of death.”

Gary Powers did keep a journal and publish his memoirs and apparently the CIA declassified accounts in 1998 followed by the Soviet Union in 2006. Unfortunately, I haven't read these.

An interesting account of aspects of his behaviour at his show trial where his defence attorney convinced him to plead guilty to avoid the possibility of a death verdict..