If your script is placed in the "Standard Assets" folder, you can't access scripts from other folders such as the "Assets" folder from that script that is placed in the "Standard Assets" folder.
Your GameManager
script is placed in the "Standard Assets" folder. The Gun_Controller_
script you are trying to access is placed in the "Assets" folder.
You have two options:
1.Move the GameManager
script to the "Assets" folder
2.Move the Gun_Controller_
script to the "Standard Assets" folder.