I find SIS are much better and you dont need to take with liquids
Most people dont really need a gel for exercise upto 1.5 hours if you have had enought breakfast/food in the days leading upto event
For marathons you are basically loading up early in the race when you dont need it to prevent issues later
I would say have every 5miles ish, but it really is trial and error , they take about 15mins to work