For information on Gentius which was well nigh impossible to find online, i turned to one of the definitive books on Early Modern Hebraism: Aaron L. Katchen's Christian Hebraists and Dutch Rabbis: Seventeenth Century Apologetics and the Study of Maimonides' Mishneh Torah. It's seriously one of my favourite books!
Another reader expressed curiosity about Gentius, which i found in Katchen's book. I've (rather poorly) scanned some biographical information that Katchen provides about Gentius, which can read here. Katchen also discusses the Abraham story, which is also available here.
Below is the article by Kohut, published in 1902:
Abraham's Lesson in Tolerance