GACUTIL is the tool available to register assemblies to the GAC.
Eg.g gacutil.exe /i C:\MyAssemblies\test1.dll
But if there are more than one assemblies to be registered, use following in the command prompt
FOR %a IN (C:\MyAssemblies\*.dll) DO GACUTIL /i %a
http://msdn.microsoft.com/en-us/library/ex0ss12c(v=vs.80).aspx