There are so many variables...
It's not so much the heat per se, it's the change in temperature and humidty that affects the gun, and it affects metal, plastic and wood components differently.
I had a Benjamin / Sheridan Blue Streak pellet rifle that I kept I kept in a side attic - not even the top top part of my house so it did not get as hot as the attic on the very top of my house. But anyway, when I purchased the rifle it had a nice looking plastic / wood imitation stock. The heat in the side attic caused the stock to develop a powdery film... it was the plastic deterioating because of the heat.
Changes in temperature cause changes in humidty which is tough on wood. I'm mainly speaking of my experience with hardwood floors, but I think it must be applicable to wood grips. A quick change in temperature / humidty can cause the wood to expand and then contract and it can split the wood.. If you have wood grips I would take them off and store them separatly.
The heat doesn't bother the metal, in fact a lot of people who restore vintage firearms heat the metal components up in an oven to get the Cosmoline off.
But... with metal it's not the rise in temperature that poses a threat, its the drop in temperature. Depending on how you have it insulated or have it wrapped, it might not be a problem, but when metal cools moisture can condense on it, which you just don't want to happen.
I think the case you have here is a good case for the bore-store or other firearms storage bags as well as the vapor-corrosion inhibiting cips that are talked about above...
You don't want to use something like Cosmoline for this - it's a hell of a cleanup job for such a short time. Even oiling the hell out of it seems messy. The storage bags with dessicants and stuff seem like a better solution