Gel or not to Gel?

Your nutrition strategy will vary depending on your exertion level, distance, heat, weight, etc.

 Here is a link to a recent topic of the week on this subject:  http://www.beginnertriathlete.com/discussion/forums/thread-view.asp?tid=122432&posts=16&highlight=nutrition&highlightmode=1#M1517355 

 However, FWIW, here is my strategy.  I'm 125ish lbs and a salty sweater (covered in salt with lines of it on my clothing post-run or ride) so adjust accordingly.

 Run  Less than an hour, nothing or sips of water if really hot or I'm dehydrated.  After that, 1 Gu every 45 minutes and water with Nuun in it as necessary throughout if it's hot or humid.  If a really long run, I bring extra Nuun for when I fill-up the water bottle.

Bike  Less than an hour, one bottle water and one bottle of electrolyte/calorie drink.  No Gus unless I've not fueled well during the day or if I didn't have a snack (nuts, Cliff bar, banana) pre-ride.  For longer rides, I take 1 bottle of electrolyte drink and 1 Gu per hour (taken by 1 shot from my flask every 30 mins).  If it's at all warm or humid, I also bring my aerobottle full of water.   If I'm doing a really long bike (> 3 hrs), sometimes I also bring a snack (Fig newtons, etc) for a pick-me-up.  Since I only have 2 bottle cages on my bike, I route past a water fountain to refill waters on a long bike and/or stash another bottle in my jersey pocket.

 For a sprint race, I take a Gu at about mile 3 of the bike.  As Kevin said, it's probably not "necessary" but does give a good attitude adjustment if you are food-motivated as well as a nice sugar boost.

 Edited to add that I prefer Gu gels because they are a little thicker (less messy) and taste better to me than others I've tried (Hammer, Cliff, Powerbar).  However, in the interest of full disclosure, I'm on team ZootGu so I have extra reason to like them better.  I do prefer the non-caffeinated versions, but that is just me.

I agree with all the above, but here is what I would add:

I haven't noticed a difference between the various brands of gel other then taste. 

Sometimes a gel is needed for fuel and other times it gives you a mental boost.  In shorter distances it is mostly mental. 

There are a lot of ways to get fuel in while exercising.  Gels are very easy to carry and easy to stomach so they are very popular.  You could also try a drink with calories like HEED or Accelerade and that way get some electrolytes at the same time.

Don't fall in the trap of thinking that you are exercising to lose weight so you don't want to eat while doing it.  What you eat while you are working out will generally get turned to glycogen (muscle fuel) and not fat.  What you scarf down at dinner because you have a calorie deficiency for the day is more likely to turn into fat. 

Here is my basic fueling:

under 1 hour and sprints: nothing

1 to 2 hours: electrolyte drink with calories or 2 gels (200 cals total)

2 to 5 hours:  Accelerade with Carb Pro in water bottles on bike (320 cals per hour).  1 gel every 45 mins on run with plenty of water that Has NUUN tablets. 

5+ hours: I do the above on the bike, but add some solid food like a clif bar or fig newtons.

Interesting thought about blood flow!

It's true.  It takes energy to digest things and that comes from your blood.  If you are going all out your body will shut down the digestive tract to put all the energy into exercising. 

note: we are oversimplifying this.