Both issues appear to be related based on what I've seen elsewhere, but often I'm unable to reload when starting up a world. If I close said world, restart my game, then re-open the world, I get an immediate tickingplayer crash relating to a "synced data key cgm:aiming" not being registered.
crash-2021-12-17_18.55.11-server.txt

Edit: The corruption only seems to occur if the game is saved and restarted while I'm holding a CGM weapon
Edit 2: It might be random? Or only if it's in my hotbar? OR if I change my keybinds???

So the issue actually lies within the underlying library (Framework). Here is a custom version that has extra logging. The JAR file is inside the zip so make sure you extract it first. Put this on your client and server and if the issue happens again, send logs for both client and server (like @twu425 did)
framework-0.2.3-1.18.1.zip

I'm getting the same error

Send your logs and the steps that happened before the crash

I got nearly the exact same error log and I got the error when loading up a world. Looking from the log, I made I blind guess that it could be my keybinds. After removing the keybind conflicts with this mod and another one the error didn't seem to happen again. It could just be a coincidence though.

Again, send your game logs and crashes.

crash-2021-12-19_19.13.59-client.txt

crash-2021-12-19_19.13.50-server.txt

Are you able to also send your regular logs? The ones that match the crash logs.

2021-12-19-4.log

Sure.

Thanks. I've looked through the logs you've sent, and it seems this is going to be a difficult issue to solve. The log is clearly stating that the keys are registered

[19:10:50] [modloading-worker-0/INFO]: Registered synced data key cgm:aiming for minecraft:player
[19:10:50] [modloading-worker-0/INFO]: Registered synced data key cgm:shooting for minecraft:player
[19:10:50] [modloading-worker-0/INFO]: Registered synced data key cgm:reloading for minecraft:player

but then the crash says

java.lang.IllegalArgumentException: The synced data key cgm:aiming for minecraft:player is not registered!

I do have a question, does this happen if the only mod installed is the Gun Mod (and Framework obviously)? The biggest problem I have is not being able to replicate the issue. I can reload worlds, rejoin servers, etc and have no crash. Usually bugs like this occur only when certain mods are install with the Gun Mod. It's possible that another mod has modified something.
Would you be willing to test a custom version of the mod that includes additional logging?

i have the same problem

Any follow up to this with the custom version of Framework?

Issue should now be resolved. Please use the latest version of Framework and Gun Mod