This returns a template from the gallery that has CategoryIds...
PS C:> Get-AzureResourceGroupGalleryTemplate -Identity Microsoft.WebSiteDeploy.0.2.5-preview
This should also return that template, but doesn't...
PS C:> Get-AzureResourceGroupGalleryTemplate | Where-Object {$_.CategoryIds -like "show"}
I tried a few other broader filters and get the same result.