Don’t expect mental focus from IF. From my own experience (14 months experience, from 18:6 to 72h fasts) and that of many others I’ve read it’s not a guaranteed thing to happen.
For your schedule, go with the time window that fits best with your daily routine. For me at first it was skip breakfast, eat small lunch and then eat a normal dinner. Most people start with 16:8 and that’s perfectly fine. Just make sure you keep a calorie deficit as well, the calorie amount depends on your weight and height, Google is your friend for that.
For gym it doesn’t matter, go when it’s convenient, fasting or not. I do heavy gym training fasting all the time with no issues.