Welcome! What’s the history in the car? Is it a car built for the Canadian market or was it imported into Canada over time? Looks to be in good shape too.
From the details others have pointed out, the previous owner(s) have made some changes to it using parts/upgrades from other model Darts. As someone else said, it wasn’t slated to be a GTS and they just ran out of 340 or 383 engines and dropped in a 318 instead. I hope you didn’t pay a higher price for it based on that story! Post up the first few digits/numbers of the VIN. A real GTS will start with “LS”. The 5th character of the VIN will be the engine code and it should be the letter F which confirms it’s a 318. If it was a GTS that letter would either be a P or an H for a 340 or 383 4bbl engine. No way that car would have a GTS vin with a 318.
As for it being rare, yup it likely is only 1 of 31. However, rare doesn’t often mean high $. Demand dives the selling price and not many folks are after a 318 4 spd Dart compared to a 340 or 383 4 spd Dart.