`[Network/VPNGateway]` deployment operation failed due to an intermittent error

## Brief
VPN gateway deployment keeps failing with helpless information

## Description
When creating a virtual network gateway for VPN Type with route-based policy, the create operation returns with server-side error without debugging/helpful information to investigate. I have tried with different SKU types and modified the customeRoute prefix value, which makes no different and all failed.

## related API:
https://github.com/Azure/azure-rest-api-specs/blob/ae227e20bcb22d83634452026f701d75bec2619e/specification/network/resource-manager/Microsoft.Network/stable/2021-08-01/virtualNetworkGateway.json#L1937

## example

gateway request payload

```json
{
"location": "eastus",
"properties": {
"activeActive": false,
"customRoutes": {
"addressPrefixes": [
"101.168.0.6/32"
]
},
"enableBgp": false,
"enablePrivateIpAddress": true,
"gatewayType": "Vpn",
"ipConfigurations": [
{
"name": "vnetGatewayConfig",
"properties": {
"privateIPAllocationMethod": "Dynamic",
"publicIPAddress": {
"id": "/subscriptions/xxx-xxxx/resourceGroups/xxx-rg/providers/Microsoft.Network/publicIPAddresses/vpnpubip001"
},
"subnet": {
"id": "/subscriptions/xxx-xxxx/resourceGroups/xxx-rg/providers/Microsoft.Network/virtualNetworks/vpngw001/subnets/GatewaySubnet"
}
}
}
],
"sku": {
"name": "VpnGw3AZ",
"tier": "VpnGw3AZ"
},
"vpnType": "RouteBased"
}
}
```

reponse error message:

```json
{
"error": {
"code": "VmssGatewayDeploymentFailed",
"details": [],
"message": "The gateway deployment operation failed due to an intermittent error. Please try again."
},
"status": "Failed"
}
```

Start with the linked virtualNetworkGateway.json definition at the referenced API location and compare it with the supplied request payload and VmssGatewayDeploymentFailed response. Reproduce the VPN gateway deployment if possible, then determine whether the REST specification needs an actionable change or whether the failure is service-side; done when the cause and next change are established.

Written by the indexing model from the issue text.