Create a new scene and make the gun, save the scene and load it when you need it.
You can preload all the guns ("gun scenes") when you start the scene with the player
var granade = preload("res://guns/granade")
var shotgun = preload("res://guns/shotgun")
...
and then show/hide/disable then as you need to.
To add a new scene you can click on the plus icon next to your current scene or at the top menu scene > new scene