In the actual fighting? Not much--according to Suetonius he helped drive one of Mithridates' subordinates from the area around Rhodes. That's about it, although the general disturbances in the Aegean area can all arguably really be considered part of the same ongoing conflict, until Pompey rolled through and put a stop to all of it, and his participation at the siege of Mytilene is notable--also according to Suetonius he was on the staff of Servilius Isauricus in Cilicia for a short period, and possibly saw fighting then. Politically, though, he was vital--it was largely through Cicero's and Caesar's support that the lex Gabinia and lex Manilia were successful, which resulted in Pompey's extraordinary magistracy