Complex sentence: Piso did not openly declare support for Gnaeus Pompeius Magnus and remained neutral but is widely believed he secretly supported Pompey but still did not forfeit the respect of Caesar when Pompey was defeated.
Split into simpler sentences: Piso did not openly declare support for Gnaeus Pompeius Magnus and supposedly remained neutral but it is widely believed he secretly supported Pompey. However, he did not forfeit the respect of Caesar when Pompey was defeated.