The world of Gate Pass is divided into 5 significant regions, although only 2 play significantly into the state of Gate Pass: Ragesia, Shahelesti, Dassen, Ostalin, and Sindaire. Most significant among these is Rageisa and Shahalesti.
As the campaign begins, Emperor Coaltongue of Ragesia has been killed, and his Second, Leska, has engaged in an empire-expanding campaign to bring all magic users under control of the Empire. Colloquially, this effort by Leska’s Inquisitors has become known as the Scourge, and has found its way as far as the neutral city-state Gate Pass.
Gate Pass itself is an oddity, surrounded on one side by the Ragesian Empire and on the other by the Shining Lands of Shahalesti. While one side or another has controlled Gate Pass in the past, decades ago citizens drove out the most-recent occupiers from Ragesia and declared independence. Emperor Coaltongue attempted to take the city, but decided the cost was too large to subdue the easily-defended mountain pass city.
More information can be obtained by referencing the Player Handbook for this campaign, available here.