Gacutil gives an error: "Failure adding assembly to the cache: Unknown Error" when adding assembly to the cache
Each version of .Net Framework contains own gacutil. If you are trying to add .net 2.0 assembly to the cache you must use .net Framework 2.0 gacutil, not 1.1 or 3.0 gacutil.
You can also manually copy assembly file to the cache. Cache is located at Windows\Assembly