The Gunpowder is one of the 3 "warmest" jackets from the UA Hunt line (Dead Calm and Rutscent being the other two).
The difference between the 3 of these is that the Gunpowder is water PROOF, the Dead Calm is water RESISTANT, and the Rut Scent has no water proofing/resistive coating at all. The only Gunpowder I have seen was a bit louder than I would care to wear while hunting, and as expected from ANY waterproof clothing, breathability is terrible. Expect to sweat while walking into your stand.
After buying a GP last season, I traded it back to UA to get a less noisy Dead Calm.
As far as warmth, these 3 are designed for stationary hunting, so they're the least breathable and warmest that UA offers. These aren't tundra-ready parkas. They're too much for early season where temps are "jacket weather," but not really enough alone for sitting in the stand below freezing. When temps get below freezing, I typically wear a hanes cotton thermal top, a north face vest, and the Dead calm jacket (equivalent warmth to the GP), and I run the risk of sweating as I walk in, but will stay warm on stand all day long. When temps get below zero, which is rare here, I add an under armour hoodie to that mix and have been doing ok. ONE parka would be that warm, but not as flexible, and typically more bulky.
Ultimately, for myself, that HATES being cold, I'd say with minimal layering, as in just a long sleeve thermal shirt and not much else, the GP jacket is good to around 30F.