While Gallio was proconsul of Achaia, the Jews made a united attack on Paul and brought him into court.For many years, there was no evidence outside of scripture that anyone named Gallio was proconsul of Achaia. In fact, there seemed to be evidence that someone else held that position. But in recent years, a "decree of Claudius found at Delphi (Greece) describes Gallio as proconsul of Achaia in ad 51". That timing matches up exactly with the timeline that Acts claims Paul was there.
Remember, when someone challenges the accuracy of the Bible, never assume they are right. Take your time, and do your research. The Bible always wins through in the end.