I find it a huge boost on race day to be taking gels.
But I have practiced one or two in the past before the marathon.
You'll need to practice the gel strategy and see what works for your. One every 7 miles is not enough as you should be able to consume 60g of carbohydrate an hour.
The fat adaption is quite a clever idea but essentially you Gould be running so slowly on the training runs that carbs and gels shouldn't be necessary. If you're running out of energy (and not doing a special long run with MP sections) you may be running too quickly.