This guide involves using an external program to hook into the game, which is what cheat programs do and anti-cheats tend to ban for. Although this game doesn't have any sort of client-sided anti-cheat that would give you a game/VAC ban, it can change at any time so you should always use your due diligence before trying these sorts of programs! But I've tested this so you should be able to follow this guide and play the game just fine.

Sizer is a freeware Windows application that can be used to change the resolution of any window (including borderless fullscreen applications) as desired. The program is completely compatible with Gunfire Reborn (I haven't tried it with exclusive fullscreen mode and I doubt it would work).

http://www.brianapps.net/sizer4/
(NOTE: I haven't tested this with Windows 11 and the program was released before it, so it may not work on W11)

Here's how you use it:

• When you first open the program, it'll take a few seconds to load then put an icon in the system tray on the taskbar. Right-click on the icon to configure it, which you need to do to add your resolution.
  
• In the configuration window, click "Add Size" and add make the new resolution the same size as your complete triple-monitor resolution (plus bezel correction).
  
• Now it's set up, all you need to do is press Ctrl+Win+Z while the game window is open and set the game to your resolution and it'll instantly change the resolution. It's a rather clean process: you can do it anytime with little risk of crashing.

This isn't all there is to do, though: there is another issue that needs to be addressed.

The second big issue with the game is that the FOV value you set in game is the horizontal FOV, not the vertical FOV. This means that, as the screen resolution gets wider, the field of view doesn't actually change, so with a 48:9 setup, even a value of 130 barely feels like 70 and can make the game feel way too zoomed in and unplayable. However, we can change this setting beyond its limits by directly modifying a config file.

The config file you need to change is located at:
%userprofile%\AppData\LocalLow\duoyi\Gunfire Reborn\usersetting.ini

The file consists of settings' keys and values. The FOV setting is represented by key 301, so look for 301=[old fov] in the file, and change [old fov] to whatever value you want. I found that an FOV of 163 at a resolution of 5820x1080 is equal to 130 at 16:9, so the value you want is gonna be somewhere between 130-163 for a conventional 48:9 setup. Save the file and you're ready to go.

With all the fixes applied, the QOL experience is completely fine. Changing the settings doesn't revert the fixes as long as you don't touch the FOV or resolution which makes fiddling with irrelevant settings very convenient, however, you do need to re-apply the resolution with Sizer every time you boot the game up.

The only issue with the game is that the HUD is scattered across the entire screen and not just contained to the centre, which demands some getting used to. Just with a small session of playing, I've adapted enough to be able to do a solo Spiritual Ascension run. Although you can't quickly see shield/health or magazine ammo, the audio cues and centred "Reload" indicator can substitute them just fine in practice. The largest obstacle is probably the very far-away minimap which is especially necessary for Spiritual Ascension, but the more you play, the more you're able to quickly glance at it and, eventually, it becomes a non-issue. I've played other games with this issue for much more time and you can definitely get used to a super spread out HUD like it's nothing.

Aside from that, the menus are completely fine: all in-game menus are completely centred and the main menu is spread across the screens, but nothing lands in the bezel correction area or awkwardly spans across screens.

Overall, playing on a triple-monitor setup with these hacky adjustments is definitely a better experience than playing narrowscreen "as intended".