I have found a pretty reliable way to install assemblies to the GAC.
First, get your hands on a copy of Version 3.5.30729.1 of gacutil.exe.
Next, copy gacutil.exe into the same folder that contains the DLL.
Lastly, open up a command console, move to the folder containing the DLL, and execute the command:
>gacutil -i assembly.dll
This way you don't have to worry about paths or different versions of gacutil.exe that might exist on the server.
However, in this particular case the extensions come with an installer that works quite well. All you have to do is run the installer on the server(s) where you want to deploy them.