Hi there. This is my first post on this forum, have been a lurker for some time now. While I'm new to this forum, I am not new to high altitude, backcountry hunting. So take all of this as one man's, free at no cost to you, experience
I have an Eberlestock J104. It will haul everything that you could possibly want to take up to altitude. It is a well designed and built pack, not as good as the J107, as is the Gunslinger. If you are looking for total protection of your rifle, than you won't be disappointed. I have handled the Gunslinger just about every time I go into my local sporting goods retailer. But I just can't talk myself into buying it. It is a top loader as you have stated. And it's only 2900 cu. inches and weighs in at 5 lbs. But it just doesn't have the handiness to load up game and equipment. It would make an excellent tactical / day hunting pack, sans carrying your game.
However, I can recommend the following pack.
I have personally hauled out two Mule Deer bucks and gear with this pack with incredible comfort and ease. Well, with as much ease as is possible with 145 lbs of gear and venison on you back. The Gun Bearer system works very well, and is extremely convenient. The added bonus of it is that it allows you to rapidly remove your rifle should game present it self at a moments notice. Unlike the GS . I can carry three days of food and all my camping gear with this pack and I still have a bit of room left over. Now my camping gear is all very light weight and compact. And I replace gear as soon as something lighter and as effective comes along. But this is one great pack. The front panel loading is terrific. Unzip, open, take a look at the entire contents of the pack. No searching at arms length to find what you may need. Internal sleeve for a 3 liter water bladder, Removable camo, and built to your specifications. The best part, 4200 cu inches and just a tad heavier than 4.5 lbs.
Lastly, I don't work for Kifaru. I wish I did, I'd like an employee discount. But I sure have bought my share of gear in search of the "right gear". I hope this helps.
Good Hunting to you,