Reluctant Gods — Paul Hutchens

The story begins in the 22nd Century when a parallel universe is discovered. This universe is an exact duplicate of our own, except that its version of Earth is in pre-Christian times. The main character, Mark, is a shy, gentle, slightly bumbling student who becomes stranded on the other Earth in 73 BC with a few 22nd Century gadgets to aid his survival. Although Mark has a gentle nature, he is faced with some extreme ethical dilemmas when Spartacus' slave revolt against the Romans breaks out. He meets a runaway slave named Rani and they become lovers. Influenced by Rani, Mark uses his gadgets to make ever more dramatic and deadly interventions in the slave revolt.