I'm not very familiar with modding weapons, but when I import the model it lies straight on the y-axis and is not perpendicular to the z-axis. First I would click on the mesh of the weapon and then select the armature while holding down the CTRL key and then select CTRL+A > All Transforms to set the rotation of both the mesh and the armature from 90° on the x-axis to 0.
Edit:
I see you haven't installed Padmee's BG3 Tools for Blender, so I have no idea whether everything will be imported and exported correctly so that it is displayed correctly in BG3.
![[Linked Image from i.postimg.cc]](https://i.postimg.cc/htqNxzbV/Screenshot-1090.png)