Never heard that gifted theory. Hmmmm.
Seen lots of kids with this problem at lunch. Very common. First time that they are eating in a loud enviornment, with all kids of noise and social opportunites abounding, and the excitement (or dread) of recess looming any second...time is so hard to pace right when you are 6. Some schools have found that this age group does much better if they play first, and eat after recess. You could try to advocate for this with your school district, but don't expect it to happen over night, but I am sure that you will find plenty of other parents with the very same concern, and groups are helpful to any cause.
In the mean time, try something that is prepackaged such that it does not "rot." Soup, or little tubs of pasta or something like that in a small thermos, even leftovers from last nights dinner in very small portions. One sandwich that even the pickiest of my children could not resist was a whole wheat hot dog bun, slathered with peanut butter, and a banana in it (like the hot dog, and the peanut butter is the ketchup and mustard, but spread out) Wrap it in tin foil so that it does not get squished. Cut it in half if this is too big. His friends will get a kick out of it, and if it looks cool, he may eat it. One more peice of fruit and whole wheat with protien, whats not to like for us too? Pampered chef has a sandwich cutter that makes the same kind of sandwich that you can get in the freezer section (cuts and seals the sandwich in a circle) This makes the sandwich smaller, and maybe if he cannot see the stuff inside, he won't think it is rotten. Again, it looks cool, and cool trumps rotten any day. You can grill these for grilled cheeze too.
Another idea is to try breakfast foods. If you can get him to eat french toast, make french toast strips with whole wheat hot dog buns (one of my tricks to get my kids to eat whole wheat) cut one bun in half, pull the bun into four pieces, soak in as much egg as you can, and serve with a small tupperwear container of syryp to dip in. I would put a string cheese package in to be sure that he gets protien. French toast sticks could go into a thermos or frezer gel container (some have the gel in them to keep stuff cold) and that way, you can tell him nothing will rot.
If he likes peanut butter, give him a small tupperwear container with a few tablespoons in it. Then crackers, apple wedges, and a banana to dip. Cheese and these kinds of finger foods are good too.
Even if you do all this, he may still send home more than he eats. I try to get my non lunch eater to eat what she left at lunch for her afterschool snack, so I pack her so that it is going to last until then. One other down side, even if they ate all thier lunch, they might still come home ready to eat everything in sight, so common. The key is to find a snack that will take the edge off without spoiling dinner. I find that a balance of protien and carbohydrates helps a lot, and homework goes better too! And last, once they get into middle school, they can eat a whole meal after school and sitll eat dinner...espeically if you have one in a sport. This issue probably won't last long!
PS giftedness is not reliablily identifiable until the end of 3rd grade. He may very well be a quick learner, but that could be different from being gifted once the pace changes from learning to read and write to writing and reading to learn as they end third, begin fourth. I would let that one be, especially when it comes to food.