You need to set-alias to the gacutil first:
Set-Alias
Name: Gacutil
Value: C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\bin\gacutil.exe

Then you can run these commands to install/uninstall dll:

Gacutil /i “E:\mydir\myassembly.dll”
Gacutil /u myassembly