I believe Gear spawns are random, although in small sample testing I saw the same bits of equipment relatively frequently in the same location. Thus, I believe there is somewhat of a bias towards certain Gear items in certain locations. In around 20 iterations, I saw a few pieces of Gear spawn more often (3-4 times, versus 1-2 times for other pieces). It may also be that my sample size was too small.
However, I was able to roll several highly desirable Gear items at the early spawn locations, so while there may be bias, I believe there is still a chance to roll "high end" Gear at the early spawns.
There does not appear to be any connection between Gear spawns and Gear types. For instance, in my experience you can roll a hat, pants, boot, or vest from any Gear spawn.
There are more Gear options than there are Gear spawns. I haven't counted them completely, but I believe there are roughly 25 Gear locations and 40 or so Gear options. Even opening every locked door, exploring every area, and collecting all the collectibles, you'll not end the game with a complete set of Gear.
Also note that some Gear is only available as pre-order bonuses, or from buying the season pass. These items you'll find early on in the game in a bar you must pass through, and can't spawn elsewhere. There is a list in this guide which names special gear for playing the Industrial Revolution preorder minigame, buying the special edition, and day 1 season pass incentives.
There's no benefit to having two of the same Gear item, and I do not believe it is possible to get a duplicate item. From my experience, once you have a piece of Gear, you'll never find it in a box again.
There are a few Gear items that seem to spawn in the same location consistently.
- Burning Halo - Found just after defeating your first Crow enemy in the Order of the Raven. It is next to a vending machine in the next area. (video)
- Spare the Rod - Found just before you reach Fink's Private Dock, in a room where they are assembling vending machines. It is behind one of the vending machines.
- Spectral Sidekick - Found in the Bank, just before the room with the tear.
- Rising Bloodlust - Found in Comstock House, in the area marked "Where we learn."
This guide states that there is one more fixed spawn gear: Health for Salts. I don't have locations for this yet, but I will update when I do.