gacutil comes with Visual Studio, not with VSTS. It is part of Windows SDK and can be download separately at http://www.microsoft.com/downloads/details.aspx?FamilyId=F26B1AA4-741A-433A-9BE5-FA919850BDBF&displaylang=en . This installation will have gacutil.exe included. But first check it here
C:\Program Files\Microsoft SDKs\Windows\v6.0A\bin
you might have it installed.
As @devi mentioned
If you decide to grab gacutil files from existing installation, note
that from .NET 4.0 is three files: gacutil.exe gacutil.exe.config and
1033/gacutlrc.dll