Socks, my son has no particular preference, other than need to be solid and match his pants. He does prefer the shorter socks. Sometimes we have extra and they have come in handy for other boys that have forgotten them.
Gym Bag - Well, my son still uses his old original bag that is falling apart. He has a newer one, but prefers the old one. The outside zippered pocket is full of holes and cannot be used at all. One thing I do is wash it out (if I remember) right before a meet - it gets full of chalk dust, and can coat his warmup/pants. If you don't have time to wash it, a damp paper towel will get out much of the chalk dust. If my son would keep his
grips in larger quart size zip lock bags (provided) and not just throw them in the gym bag this wouldn't be a problem. Plus he uses his meet bag for everyday use too. Just what he likes. Same bag he has had for 7 seasons (next season will be 8).
Shoes - Never been an issue. My son just takes his shoes off and gives them to me at the beginning of the meet. I stick them under a chair. Even if I move around, I leave the shoes. Never had a problem with finding the shoes after the meet. He has those sliders but he never wears them.
Excellent recommendation to label everything, particularly the warm up and the pommel pants and shorts. We have had pommel pants disappear (and they were labeled) at meets more than once.
If your uniform is white shorts, might want to get white underwear. My son's normal uniform is black, so no big deal, but at the last meet it was funny to see some kids with white shorts with a variety of different brightly colored undergarments visable (not little kids, this was at nationals). We hadn't packed any white underwear or socks so picked some up at the local Kmart.
Cash - entry fees usually around $10, plus bring $$ for food for yourself or pack snacks. Although, some meets are starting to take credit cards, I wouldn't count on it.
Snack for gymnast - This is important. Many kids get tired for the last couple of events so having water and a small snack for a boost of energy to eat after the 3-4th event is very helpful so they don't run out of energy and crash. Something small and healthy. NO gatorade. I will never forget the meet that the host gym provided gatorade in coolers, we were wondering why the boys were so thirsty and kept drinking from the coolers over and over and over. Well, about 1/2 way through the meet my son was spinning around on the floor, and he wasn't the only one! It was pretty funny actually. We have video of that meet.
Hair - If your gymnast has hair that might get in his eyes, the judges may not let them compete. We had a judge tell our coach a gymnast needed to do something about his hair getting in his eyes, so his mother got scissors and chopped his bangs right there (a hair tie would have worked). My son used to wear his hair pretty long, so we used to do quick and not always straight bang trims right before meets. Or use a lot of hair gel.
Some meets don't have great seating. Some are just bleachers with no chair backs. So if you have stadium seats, if you leave it in your car you will most likely need it for at least one meet during the season. ( I forget this myself a lot and regretted it).
Video camera - charge the battery the night before. And don't turn it off when the event is over, some of the best moments are of interactions after the event. I don't always do this myself, actually don't video every meet, sometimes it is just more fun to go and relax and just enjoy the moment. Or you could trade off with another parent too, so you still have video but take turns recording.
I must say the first few seasons of competition were more stressful than now. So try to just enjoy it. My best memories are of team mates that really struggled that finally got skills at meets, or funny things that happened (like when my son did his whole P-bar routine with his pommel pants pulled up to his knees and the judge was laughing during the whole event). It's the funny things that I remember, not the ones were everything went perfect.