You might not need to eat before your workout. For a short workout you can probably have carbs afterwards. For a long workout take some carbs after an hour maybe. Be sure to carry something with you in the gym. A sugary drink is probably best as your digestion may be slowed while you are exercising hard.
It's probably advisable to reduce your insulin slightly, ahead of exercise and for a while afterwards, up to 24 hours depending on how strenuous a workout.