Amanj i start off with shotbloks and then have a gel every 45-60 minutes. I normally end up running at least 5 hours for a marathon so have about 6 on me in different flavours. I use the sis gel ones and have practiced them before in training. Its better to do this before the day in case they dont agree with you.
i usually have some water when taking a gel. Although they do have water on the course i carry some with me as ive been in a situation where water supplies have run low. Maybe the others who have done Paris before can advise if this has ever happened?