I have a Galaxy S3, and it gets really hot as well running GPS with the screen on the whole time. It also tends to start locking up the phone (though likely less of an issue with the G3) and burning data, due to regular map downloading. Now, I simply turn off the screen on long stretches of highway. Google Maps also now has an option to turn the screen off in between turns, but I do it manually. It saves data, saves battery, and DEFINITELY keeps the phone cooler. Plus, if you don't realize you're coming up to a turn, it will still read out instructions with the screen off.