Hellgate, or Hellgates is the name of a pass in the Clark Fork of the Columbia River, just East of Missoula, Montana, and by extension, is sometimes used to refer to Missoula itself.
The pass is between two large hills, Mount Jumbo and Mount Sentinel. If you wanted to get from the Western parts of Montana to the Eastern part, you would probably end up going through there. In fact, I-90 goes through there. And that is how the pass got its name.
Before the area was settled by Europeans, but after they had brought horses, groups of Salish, Coure d' Alene and Spokane 'Indians' would supplement their largely Salmon based diets with Buffalo. Once a year, they would ride from the mountain valleys of Western Montana to the Great Plains of Eastern Montana to hunt buffalo. And, of course, they would have to pass through the narrow pass of the Clark Fork river.
The Blackfoot Indians, a group that ranged across Eastern Montana, were either not too happy about the intrusion, or just wanted to loot the hunters. So when the hunters went through the narrow pass, the Blackfoot would often ambush them.
In fact, this happened so often, that before too long, the pass, which in certain places was only a hundred yards wide, became so littered with corpses, that it begin to take on a rather hellish appearence, as dozens of bleached skeletons covered the ground. When the French arrived, they gave the area the name Port d' la Infierno, which was translated into English as "Hellgate". The word that the Salish and their allies gave to describe the place Isul, meaning horrible, gave Missoula its name.
As a side note, certain members of the Blackfoot tribe later migrated to Staten Island, where they formed the Wu-Tang Clan.