Doesn't the REST API contain Virtual Network Gateway yet?
Got this error in AzureResourceManager mode, and a similar one when deploying an ASR template containing a gateway.
PS C:\> Get-AzureVirtualNetworkGateway -ResourceGroupName ResourceGroup1
Get-AzureVirtualNetworkGateway : InvalidResourceType: The resource type could not be found in the namespace 'Microsoft.Network' for api version '2015-05-01-preview'.
At line:1 char:1
+ Get-AzureVirtualNetworkGateway -ResourceGroupName ResourceGroup1
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+ CategoryInfo : CloseError: (:) [Get-AzureVirtualNetworkGateway], CloudException
+ FullyQualifiedErrorId : Microsoft.Azure.Commands.Network.GetAzureVirtualNetworkGatewayCommand
It is included in the documentation here: msdn.microsoft.com/en-us/library/azure/mt130667.aspx