What do you do on the days when you feel like your brain wants to go to the gym but your body feels heavy and not motivated?

Depends on the day! You probably know your mind and body best. Do what feels right. Sometimes a vigorous exercise can wake the body up. But also, sometimes rest is exactly what the body needs. And then there will be days that you can satisfy you mind and body with a compromise by going for an easy walk, doing an easy yoga session, etc.