I know that you asked about a specific product and not for advice, but....
Probably none, that would adequately do the job.
This is the left Achilles heel of the Gauntlet (the right is its length and weight) you really can't get away with using a soft case. If the barrel gets any pressure on it, it will shoot off. With that said, I have mine in a huge hard case and it has always been first shot accurate and has never lost air. I can rely on the accuracy coming out of this case. I don't know of a soft case that you can throw in the back of a pickup, or even just drop to the ground unevenly that I would absolutely bank on my zero remaining. I suppose it depends on your use of it, if you are just looking for a sleeve to store it in, anything will work. If you are traveling anywhere, I would strongly recommend that you don't hamstring your zero on a soft case. Gauntlets are amazingly accurate for how they are made but they are not made even close to how, for example, an FX Crown is made, and with a 700mm barrel on it, I wouldn't put one of them in a soft case either. At the length these things are there is a lot of leverage to deal with. Keep reading and you will come across articles here where people talk about the poi shifts from just using heavy moderators, which is true, and nothing compared to knocking into a doorway, or a tree, or a vehicle with stuff piled around them. It is very inconvenient trying to travel around with my Gauntlet because of the size of my case but it does work. My ultimate solution to the problem of travelling with it was the same as everyone else's on this forum, I got another airgun, a bullpup. Good luck on your search.