Can this go in a garage that is insulated and has drywall? It may get down to 40 degrees in the winter and maybe 90 degrees in the summer.

Infrared saunas do not typically perform well in ambient temperatures below about 65 degrees Fahrenheit. The garage location would be fine, but you would need to warm the garage air when you wanted to use the sauna. I would recommend warming the garage to about 70 degrees Fahrenheit when you wanted to use the sauna.