'He' was obviously in a position of power. So he had the power to arrange for those criminals to be sent to jail. By the way, what exactly is giving you trouble here? Also, could you please give the source and some context/background. Where does the sentence come from?
Yes, you are right. He sent them to gaol*. - he was the one who sent them to gaol. How he did it, we don't know. Presumably, he was the judge or someone who could directly order them imprisoned. He had them sent to gaol. - he did not directly do it. He arranged for this to happen, presumably from a position of high power.
* Pay no attention to this, I just like this extravagant spelling of jail.