my brother works part time as a fireman. according to him, its all about eliminating fuel sources, and examining how a house burns.
heat rises. a basement of a fully engulfed house will be receiving somewhere in the neighborhood of 40-60,000 gallons of water. a portion of that water will evaporate quickly, but the net affect is that the basement stays relatively cool.
additionally, there's not much fuel source in an outside corner of a garage. a fire there would quickly consume its fuel, and move on.
i think i remember my brother saying that the average house fire temp is around 1300 degrees, in the center of the heat. locating a safe in the basement or garage would most likely not reach these levels, or if so, would not be sustained for long.
i certainly think 10 gauge steel is sufficient. that's going to make one heavy safe.
my hunting partner purchased a champion safe recently, and that's the one i'll be getting sometime soon. apparently, the family that makes liberty safes had a little falling out, and thus, champion was born about 10 years ago. they have all the features of a quality safe - double locking bolts on all 4 sides of the door, a re-locking mechanism should someone try to tamper with the dial, and heavy gauge steel. additionally, their warranty is second to none in the industry - if anyone trys to break into it, take it, or it burns in a fire, they replace it for free. not bad.
the one my hunting partner got is the Champion Victory 50 (41"W X72"H X28"D). it weighs 1250 lbs empty. i highly doubt that any theif is walking away with that, even though he has it in the garage.
depending upon the safe that you get, you might be limited as to where you can put it anyway. most places i've checked into will deliver it to your driveway, not in the house. getting one into the basement is sometimes impossible, and in today's age, it will probably cost you $200+ to have a moving company (if you can find one that will do it) put it in the basement for you. obviously it all depends on the size of the safe, but ....
i think we need to be realistic about what a safe provides - that is, a secure area for our valuables. i've talked to a number of gun dealers, and firemen who've inspected safes after fires. i think the difference in burn ratings on a safe will make the difference in whether you can read serial numbers and such. any fire, unless contained quickly, will likely (at a minimum) damage the surface of guns in the safe. that's what insurance is for, i guess.