I think the most obvious reason to not use baby food is that there is a lot of added sugar in most of them. if you are good at reading into labels and nutrition facts and can find a nice food without added sugar or artificial ingredients, it seems like it would be a good idea. However, many baby foods are over processed and have garbage like artificial preservatives and colors added to make them last longer on the shelves and appeal to the parents eye. They also probably have a lot of good stuff taken out- like pulp which basically equals fiber- and I think that you should leave that in.
What I do for gutload is get a whole bunch of veggies and fruits together (think blueberries, strawberries, whole mango, several different varieties of whole squash, kale, collard greens, mustard greens, whole yucca root, prickly pear leaves, apples, a whole plantain, whole kiwis, whole oranges, whole limes, carrots, whole zucchini, whole yams, dandelion greens, endive, parsley, whole eggplant, brussel sprouts, whole sweet potatoes, green beans, whole (no top) pineapple, and lots of others that I can't even think of- it is an all day project) and wash them off, blend them together and dole the mix out into ice cube trays and put them in freezer bags. I can then pop out one cube at a time for my gutloading containers and it seems to work fine (and has for the last year) When I say "whole" I mean skin and all! Just wash well and use organic when you can. The more variety, the better. I also use a nice dry mix that I blend with a coffee grinder. It has lots of stuff- some of it is bee pollen, wheat germ, 10 grain cereal, sunflower seeds, kelp powder, dry egg yolk, hemp seed nuts, spirulina, irish moss powder, dry yucca root, alfalfa leaves, lecithin, and lots of other stuff that I can't think of off the top of my head. I offer both mixes- the wet one has enough moisture for the bugs so you don't have to provide water gel. I put in enough for a day or two and clean often- it is prone to mold so only put in however much your bugs can eat that day.
If you have problems finding some of these ingredients, check the frozen section of the store. Read the ingredients and make sure nothing is added- that it is just the fruit or veggie. This is how I get berries and some of the other fruit year round. You can also check health food stores- I work at one so all of this is available to me.
Hope that helps!