How to GAC in Win8

In .net 4.0 Microsoft removed the ability to add DLLs to the Assembly simply by dragging and dropping into C:\Windows\assembly\.
Instead you need to use gacutil.exe, or create an installer to do it.

To use gacutil:
Start -> programs -> Microsoft Visual studio 2010 -> Visual Studio Tools -> Visual Studio Command Prompt (2010)
Then use these commands to uninstall and Reinstall respectively. Note I did NOT include .dll in the uninstall command.
gacutil /u myDLL
gacutil /i "C:\Program Files\Custom\myDLL.dll"

To use Gacutil on a non-development machine you will have to copy the executable and config file from your dev machine to the production machine. It looks like there are a few different versions of Gacutil. The one that worked for me, I found here:
C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\gacutil.exe C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin\NETFX 4.0 Tools\gacutil.exe.config
Copy the files here or to the appropriate .net folder;
C:\Windows\Microsoft.NET\Framework\v4.0.30319
Then use these commands to uninstall and reinstall respectively
"C:\Users\BHJeremy\Desktop\Installing to the Gac in .net 4.0\gacutil.exe" /u "myDLL"
"C:\Windows\Microsoft.NET\Framework\v4.0.30319\gacutil.exe" /i "C:\Program Files\Custom\myDLL.dll"