Using gym equipment like the treadmill, bike or cross trainer are good ways to exercise, raise the heart rate and generally build up your fitness. Most gyms are open now with a reduced number of pieces of equipment as well as changes to the way the gym space is used. Its a good idea to think about whether this is the best place to get active if you have a single ventricle heart condition as you will be inside and close to other people, outdoor exercise would be a better choice as it holds fewer risks of catching Covid-19.
Weights, especially upper body weights or too much upper body exercise is not great for someone with a Fontan circulation as building up too much upper body muscle creates resistance for blood flowing to the head and then returning to the heart and lungs. General upper body fitness is fine.