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.