Using MSDeploy gacInstall to uninstall or force overwrie of GAC assembly

I have a set of assemblies in the GAC that I'd like to update using MSDeploy and a set of files.

If I perform the following:

msdeploy -verb:sync -source:gacInstall="C:\test\assemblies\MyAssembly.dll"
    -dest:"MyAssembly"

Everything works fine IF the assembly is not already GAC'd.

However, if the assembly is already GAC'd, then the file is not actually updated it seems.

I have tried:

msdeploy -verb:delete -dest:"MyAssembly"

And this does not seem to work.

So the question is whether there is a way to uninstall a GAC'd assembly or force an overwrite using MSDeploy.