There isn't a dress code, but if I were to describe it, i'd say "as casual as it gets".  You might see a company polo now and again, but thats about it.  

"as casual as it gets"

Does pijamas count as casual?

For the record, that was a joke.

Nice

More safety/personal comfort than etiquette, but maybe don't wear open-toed shoes in the vendor hall. It can get pretty crowded and toes can get stepped on. 

Most of the time, I found shorts and a t-shirt fine, but some rooms can get cold and the whole place cools off at night, when there's less people and sunshine. 

Throw a stick of deodorant in your bag, so you can step into a restroom and re-apply as-needed. 

If you're wearing a backpack in the vendor hall, try not to make it larger than your average-sized student backpack. It will be a lot cooler and it won't get in people's way. I use a mini-backpack for snacks+water and that stick of deodorant, then stuff a couple of large nylon shopping bags in it for my game-buying needs. That way any bulky packages I'm carrying around are in front/beside of me, not behind me. It's easier to manage in the crowd and less prone to theft (though I'd be surprised if the latter is a real issue).

Black gamer tees are excellent camouflage and hide sweat stains pretty well (it's indy and more than a few of us are people of mass. Practice daily hygiene (showers) it's Indy sweat is going to happen no matter the amount antiperspirant is applied. You won't be the only one.  

Spare insert for shoes help as you can then rotate and dry out a set if you don't want to or have spare set of walking shoes for con. I carry a set of flipflops/sandals for walking from hotel to food and back.

Most of the responses have been practical what to wear advice. 
I think there is a restriction on costumes that appear to be current law enforcement or military.

God I don't miss her being there any more. Got mad because others didn't share her point of view.

I'll second bringing a sweatshirt or pullover jacket. The ICC has some surprisingly cold areas. Also, consider that you may want to go out to a decent dinner at a bit more upscale eatery at some point.
 

You will do a TON of walking.

Wearing flip flops for four days and walking for miles really, really hurts.  Bring tennis shoes.

Also, I've found it's really useful to bring some kind of cream or powder to put on the inside of my legs.  I'm not fat, but I still wear that area raw from all the walking at GenCon.  It helps a LOT.

I don't wear flip-flops, but I often wear more secure sandals and I've never had an issue except my feet getting cold in the ICC (of course, this is going to jinx me and for sure someone will step on my foot this year). It really comes down to Know Thyself for footwear- if you stand/walk around all day in your everyday life, you will probably be fine with whatever is normal for you, but if you know the amount of standing and walking you will do is way outside your norm, then yes, be sure to bring supportive shoes or whatever else you think you'll need.

Unless you run really warm, I also agree about bringing a layer for the ICC; they blast the AC so it will feel normal in the really crowded areas, but once you sit down somewhere less congested, it can get cold. 

Overall, I don't think the businesses/restaurants directly around the ICC really care if you're casual, even the upscale places. I've only had one experience where I thought our group was being judged for wearing jeans and t-shirts, and I think the issue was with that particular server, as we had been to that restaurant before with no issues. 

I've never had an issue with dress code at downtown restaurants but i've had a glance or two from locals when i've been to a few restaurants, Fogo de Ciao and Oceanaire come to mind. I end up packing 1 nice travel shirt with a collar but that's me and less public dress code.

Tks all for the tips!

Considering some of the cosplay I’ve seen, there does not seem to be anything close to a minimum dress code. There have been times when I thought I could see what religion they were. ;-)

It your flying you might consider taking underwear and socks you are willing to throw away.  Or disposable underwear? That will give you more room going back home.

OK I have to ask the OP... was this meant as a joke or have you never been to Gen Con before? (no disrespect meant)

Dress code? never seen one... I think it 2005 or 2006 i came across a group of people outside the hall, there was a young lady laying on the floor screaming in agony. The bar bell nipple piercing she was wearing got entangled in the chain mail bikini top, bikini top being a relative term, that she was wearing. This happened before the guy you see every year now with his sigh that says "Free cos play " repair. 

I must say they either cracked down on the booth babes, but the only dress code I've seen is no guns or toy guns that look real. Of course that hasn't stopped the guy with the true functional Braveheart sword :)

Never really be there. flying from Brasil for the first time.

That is why i have been asking a lot.