The primary ingredient for the regular infant formula is "nonfat milk". The primary ingredient for the "gentlease" and "sensitive" formulas is "corn syrup". Seems to me we should be going with the regular infant formula even if that means a little gas. Am I wrong for thinking milk is better than corn syrup?
There's a big difference between sugar and more complex carbs like lactose. See http://www.foodreactions.org/intolerance/lactose/absorption.html for an in-depth explanation.
Lactose will be converted to only glucose, and this will only happen in the small intestine, at a certain rate. Corn syrup however is glucose + fructose, which will hit the blood as soon as it can. Fructose has several issues, see http://high-fat-nutrition.blogspot.com/2012/05/on-glut5.html for some background (sorry not really an overview link but better than the Lustig lecture).
So the blood sugar response between the two types will be different, with the corn syrup one unlike what babies will experience on mothers milk, and the fructose in corn syrup may cause stress on the baby's body, it is a sugar babies are normally not exposed to until they eat fruit and even then not in very high volume.
Add to that the possible antinutrients that are in corn and in the syrup, and corn syrup becomes a cheap but in my eyes criminal choice.
Mind you I think http://www.westonaprice.org/childrens-health/recipes-for-homemade-baby-formula is the best formula so I'm probably biased :)
Note that nonfat milk could reasonably be called protein-fortified sugar water. The nutritiondata entry for milk indicates that one cup of nonfat milk is 12g sugar, 8g protein, 222g water (and 3g other). Compare the nutrition information for Gentlease versus regular Enfamil and you'll find that they are quite similar. My experience is that formulas are very similar in terms of nutrition. So, if I were you, I'd go with whatever formula works best for your baby that fits within your budget.