April- if you do an easy 20 sometime this week, you should be fine. Especially if it makes you feel more confident for your race. Your base mileage is sufficient that you should recover as long as you go easy. But no more flag football till after the marathon, ok?
Tapering- I would disagree with Sasha and say should should taper, even if you aren't doing the marathon "competitively". Keep your workouts the same as before- do a little fast stuff and the rest easy. Keep the intensity the same, just reduce mileage. You don't want to make the mistake of 2 weeks of very slow, sluggish runs- mix a little fast stuff in, just keep it short.
Gatorade- if you did that in training, then do it during the marathon. If you wanted to try gu's during the training the next 3 weeks, and if they work for you, then you could have that in case of emergency during the race.
Since this is your first marathon, going out slow (8:00 pace) for at least the first half is a good decision. Don't let yourself get sucked up in the excitement at the start and run a 7 min mile or anything. The slower you start, the better you will likely finish towards the end. And you can always speed up after the 13 mile or 20 mile mark if you feel good, like Sasha said.