the Fifth Age of Ert
This is a modified d20/AD&D running set in a world with guns and magic. More of a Boothill / Steampunk campaign than traditional D&D. Magic is more common in the world but less powerful. There are no gates or teleports but instead carriages and railroads. All of the nations of the world are from Earth (Rome, Germany, England, etc…) but on a different map.
Wikipedia is a source for most history, with adjustments happening after the 1500s. Starting with the defeat of the English Navy by the Spanish Armada…
A History of Ert