1.To install this mod : Requires Mod the Gungeon. Place the BasicGun zip file into the mods folder of your Gungeon's steam directory. Also make sure to delete the auto-generated mods.txt file inside your mods folder if there is one.
2.In order to spawn the basic gun, use the command kp:basic_gun
I also recommend getting Dnspy-net472: https://www.youtube.com/watch?v=8K9AmIYtdcY&t=11s
Dnspy is a .NET debugger and assembly editor.
Aka, it lets you look into the code of Gungeon and gungeon mods.