I have a problem with my custom character. Firstly, I created it in blender and added the bones in blender as well. Once I did it, I imported the character in Roblox with : Avatar Importer > Custom.
Wait, why does your character only have a HumanoidRootPart? Is the whole character just one part? If yes then you may need to add attachments yourself and align the gun accordingly.
Create attachments on the HumanoidRootPart to match the position of the arms and legs or whatever you need. The problem is, I’m not sure if the attachments will move since your character is made of bones not of multiple parts.
Okay then yes attachments most likely won’t do the trick. There’s a post of a few months ago that asked a different question but with the same goal, someone recommended using RigidConstraints to attach accessories or in your case a tool to the character, although I don’t know how you’d code it.