You need to change current folder in command line to <Windows folder>\Microsoft.NET\Framework\v3.5 before running gacutil. For example, if Windows is installed in C:\Windows then you should first execute in command line:

Yes I know I did that I have basic knowlage of how the command line works. And it does not recognise it. Also there is a framework64 folder. Maybe I can try that folder?

Sorry, may bad :/ Gacutil is also a part of Windows SDK, as Fusion Log Viewer. I've attached zipped EXE.
Have you tried with XNA Game Studio yet?

XNA Game Studio requires me to install Visual Studio 2008 Express and I'm having a hard time installing that. This is what gacutil says:

Microsoft (R) .NET Global Assembly Cache Utility. Version 3.5.30729.1
Copyright (c) Microsoft Corporation. All rights reserved.

The Global Assembly Cache contains the following assemblies:

Number of items = 0

Seems XNA Framework does not install properly - the assembly "Microsoft.Xna.Framework" should be placed in GAC during installation. Did you run its installer with administrative privileges?
You can also try to install this assembly in GAC manually:

• Extract Microsoft.Xna.Framework.dll from xnafx30_redist.msi. You can use 7-zip for that.
• Place it in the same directory where gacutil resides.
• Open command line with administrative privileges (run as administrator), and execute:
  
  Code:
  gacutil /i Microsoft.Xna.Framework.dll
  

No I didn't run the install with admin privilegies. There seems to be no Microsoft.Xna.Framework.dll in the install. There are Microsoft_Xna_Framework_dll_GAC and Microsoft_Xna_Framework_Game_dll_GAC inside the archive which are recognized as a "File" type i.e unrecognizable.
Should I rename them? Maybe it is a corrupt install?

First try to run XNA Framework installation with admin privileges.
If it does not help, follow instructions for manual installation. Extract Microsoft_Xna_Framework_dll_GAC and rename it to Microsoft.Xna.Framework.dll.

Hmm... Both solutions lead to the same result even though I am running them as admin. This is what it said in the console when I tried manual install:
Microsoft (R) .NET Global Assembly Cache Utility. Version 3.5.30729.1
Copyright (c) Microsoft Corporation. All rights reserved.

Failure adding assembly to the cache: Access denied. You might not have administ
rative credentials to perform this task. Contact your system administrator for a
ssistance.

Try logging with 'Administrator' user (which is setup at OS installation), and also running command line with admin privileges (right click, 'Run as administrator').

How do I do that? I see only my profile and it has administrator rights.