Have used lucozade sport ones in the past which are quite gloopy but recently tried shotz and think I'll switch - much easier to get down.
You should be taking gels before the last 6.2 as it takes a while for the carbs to get into your system. One every 5m is plenty ie start,5m,10,15m - one at 20m will not be much use as you will be nearly done by the time it kicks in - better to scoff a few jelly babies for a quick energy boost in the last bit.
Most important bit is to practise on your long runs, find out what works for you and stick to the plan on the day - good luck!
Oh and tapered rested, adrenalin on the day, grit and detirmination etc all true too!