When told the temperature was going to drop below freezing, I checked my car for antifreeze, made sure there were no liquids inside the cabin, etc. But now, when I start the car, I smell gasoline and it lasts for a few minutes after driving the car. Did I not have enough antifreeze, causing the water in my car to expand? How would that cause a possible fuel leak? 2003 kia spectra, if it's important
Hyundai/Kia are known for having issues with their EVAP control systems. I owned a 2007 Hyundai Tiburon and the EVAP solenoid was malfunctioning. It made a pretty strong fuel smell because of the leak it caused. Cold weather can cause some of the rubber hose and emissions tubing to harden and crack under stress. Another thing to keep in mind is that when plastic or rubber is taken from near freezing to engine operating temperature, the material can crack or break down quickly. The process of hardening steel using temperature treatment is similar to the stresses of what happens to rubber and plastic lines when they are cooled down and heated up quickly. They will become less pliable over time and incremental movements of the car and the engine can greatly effect the lines.
You have "Freeze Plugs" on the engine and intake manifold. These prevent the expansion of frozen liquids from damaging your engine and other components. These should break loose from the engine upon expansion, but they can malfunction and not break loose when they should.
One thing to do is monitor your fuel trims that the computer provides. Kia/ Hyundai are usually generous about the information they provide via code scanners. Your long term fuel trim on that Beta II engine should be within 10 and -10. As cars age the fuel trims can dip into an unacceptable state. Anything over or below the "10" mark is considered an issue.
If your EVAP or fuel system in general is malfunctioning, you'll notice that your fuel trims will try to drastically over correct for the issue. If you have a vacuum leak you will end up with a lean condition. This means that too much air is siphoned into the system, so the fuel to air is not correct. You will most times, not get a diagnostic code unless the mixture is off by A LOT. I don't think you did anything wrong here, I think that the drastic weather changes may have assisted in something "breaking" on the car.
You need a diagnostician or a scan tool to see if the engine is working properly. 50/50 antifreeze is usually the best to use, but if you're in an area that gets REALLY cold, you may want to go 30/70.
I would like to provide a more thorough answer, but theres only so much I can tell you over the internet without actually hooking up diagnostic equipment =(