Don't really know much about the GT's, but of my understanding, all of the 3100/3400 engines are effectively the same, just the 3400 is larger displacement of course.
From google results
A 170-hp, 3.4-liter V-6 engine is standard in the SE2 and optional in the SE1. A 175-hp Ram Air V-6 with cold-air induction and a lower-restriction exhaust system goes into the GT and GT1 and teams only with the automatic transmission.
so there's 5hp difference, probably so little difference in the tune you'd never feel it.
I'm a bit of a Toyota fanboy, but the 1MZ 3.0L camry engine (not a sports car at all) puts out around 190hp and is quite a lot smaller engine, 6 bolt main, 10.5:1 compression =).
For the coolant thing, sounds like it needs to be burped and get the air out of the system.
Scanners read things differently, normally you have to tell it the vin digits and then it picks that stuff up for trim and such. Pretty sure it's just reading the vin number in the computer to flag it as a GT or not. In the computers I've been into, there's only 1 location the VIN was stored.
Fun little side fact, the older Grand Am's from the early 90's with the quad 4 2.3L 4cyl engine had an option for a high output that made 180hp, more than that 3.4L and a whole lot smaller. Those things zipped around pretty nice with the standard engine in them (160hp if I remember right). Trim names are basically just marketing jargon, just something to extract more money out of the buyer in some cases. Like Toyota is guilty as well with their "American Edition" aka base mode car with a dealer installed security system.
Anyway, not trying to bash your car or anything, just I'm not really impressed with GM's, even the newer 3.5L doesn't make that much more power (200hp). That's more of the American way though, make the engine bigger and that's the cheap/easy way to make more power. Power scales with displacement lineally, aka that 2.3L 180hp engine scaled to a 3.4L would be 266hp if all else was equal. Less power generally means more reliable, so fair chances that's why they don't push the power too much.
