I did GNR in 2012 with a 3 year old and I had hardly exercised at all until 6 months before. It's possible! Amazing atmosphere and great route, I hope you love it.
A lot of info about gels and fuelling is aimed at blokes whose metabolism is quite different (I'm assuming you're female!). I have a tendency to IBS and need to remember to drink lots of water when using dense energy products like gels or protein bars. But just try things out and see what works. I wouldn't use gels for less than an hour's run. I would have a protein heavy meal (cheesy omelette is my go-to) and coffee a couple of hours before, and a banana and water half an hour before. Then I'd have fruit/nut bars and another banana ready for immediately afterwards, and a rehydration drink.
If you can, follow a training plan, they really work (I think there's a couch to 10k, as you're already able to do 5k). Or join a running club (8 know this is really hit and miss depending on the type of people in your area and your available time!), or seek out a trainer (££ but really impactful).
And you will reduce your injury risk and improve your comfort and times if you do strength training too, working on your legs one at a time, and your core. Squats, split squats, calf raises, woodchop, deadbugs, bird-dogs, windmills, and box jumps.
Can we talk about bladder weakness too? It happens to EVERYONE and it should not be embarrassing. I have a wet mark in the (professional, posted on the race website) pictures from a 10k I did earlier this year... The one where I set a pb of 57 mins, boom! I do wear 'period pants' but to be honest they're not very effective. Dark shorts/leggings are the answer, and a shower after.