You don't even need to brown your chickens, go to the local grocery store and buy 6 plain rotisserie chickens ( you may have to pre-order them) debone them and put them in your gumbo.
Here's a thread I made last year.....
Here's what we had in it.....
We ended up with a lot left over..... probably a little less than half the pot, so if you can do an 80 quart pot for 100 people it will be more than enough.
As far as Roux goes, my mom makes enough roux to fill a quart mason jar. Uses what she needs and puts the rest in the fridge, it last a month in there without issue. My wife does the same thing and we never had a problem but we do use it up fairly quick.
One more tip, one time i was using my wok to cook some stir fry, I went a little to long with the onions and they browned really quick! now, when I'm in a hurry I use a wok to brown my onions. Just need to make sure you stir them constantly and don't take your eyes off them. You can brown 2 onions in under 10 minutes.