How do I import 3d models from blender into my map as a gun?

This is a bit old, but you need to export your Blend as an FBX file. And then import that into Unity.

From there, you can follow our documentation and/or use the packaged example assets to create the prefab and other stuff for your Gun asset in Unity, and use the imported FBX for your model(s).

https://docs.smartlydressedgames.com/en/latest/items/gun-asset.html

Then you can export your Unity assets as a master bundle, create your .dat file(s), and test your mod.