As we write walkthrough's on how to do various things with Web Deploy or other tools, we often say to try this or that with gacutil.exe but I always forget where the tool lives. Since I re-image test machines all the time with various things installed, I forget this stuff (perhaps I'm the only one).
It's included with Visual Studio (as part of the Microsoft SDK) and the .Net framework (v1 and 1.1 only!) So it might be in one of the following (or more!):
%programfiles%\Microsoft Visual Studio .NET 2003\SDK\v1.1\Bin\gacutil.exe
So either you'll need to download this from the old .net frameworks, or you can install it as part of VS. (Haven't checked if it comes with the Express editions.)
Since none of these places are probably on your path, you'll need to either update your path (temporarily or permanently), or just run gacutil.exe from wherever you find it.
Hope that helps!
BTW, here is the command-line reference: http://msdn.microsoft.com/en-us/library/ex0ss12c(VS.80).aspx