Permatex says this: "Silicone dries in 1 hour, and fully cures in 24 hours, at 70ËšF - 50% relative humidity. Allow more time in cold weather."
The rule of thumb from Organic Chemistry was that the rate of reaction doubles for every 10C increase in temperature.
So, working it backwards, room temp/70F = approx 20C. You're curing at, worst case, 0C, in your unheated garage, so, let's say 1/4 the reaction rate for curing.
So, worst case, again, you would need to let it cure for 4 hours to dry, and 96 to fully cure, but here's how I view it, you don't need to be "fully cured" to start the engine unless it's a really challenging application. Once you warm the engine up, and it runs for a few minutes, that silicone is going to be fully cured. If you avoid putting pressure on the gasket during that "fully cured" period, then you're fine.
TL;DR - You're fine.