i'm doing a 4 day 300 mile bike ride?

I heard of that gel. I've seen many riders eat/drink it and they say it just give them a shocking tangy taste. I would just stick to a Power Bar that has plenty of protein to stimulate you muscles. GU Energy Gel is not needed.

GU packets are essentially 100 calorie packets of cake frosting. 100 calories will fuel you for approximately 2.5 miles.

I carry a few in my jersey pocket during long races, but for a supported bike tour, you'll have periodic fuel stops. It wouldn't hurt to have a couple on you in case you run out of gas in between, but it shouldn't be necessary. Your main focus as a noob should be keeping yourself hydrated, meaning you need to take on water at something like 1 liter per hour. That depends on weather and your sweat rate of course, but its a good starting point since you obviously haven't had it tested..

Where I live, the place is small and there are food outlets almost everywhere

We can stop and buy food so easily, there is no need to carry food ration

Averaging 75 miles a day, we do not need abnormal/unusual food for energy