The list of assemblies stored in the Global Assembly Cache (GAC) can be found in the registry under the HKEY_CLASSES_ROOT\Installer\Assemblies\Global key.
If you want to get a list of the assemblies registered in the GAC using PowerShell you can use this snippet:
|New-PSDrive -Name HKCR -PSProvider 'Microsoft.PowerShell.Core\Registry' -Root HKEY_CLASSES_ROOT|
|Get-ItemProperty -Path 'HKCR:\Installer\Assemblies\Global' | Get-Member -MemberType NoteProperty|
The first line registers a new drive called HKCR in PowerShell that maps to the HKEY_CLASSES_ROOT in the registry. This is required because, by default only the HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE registry hives are registered as drives in PowerShell.
The second line just gets the list of registry properties in the HKEY_CLASSES_ROOT\Installer\Assemblies\Global key and displays them.