Azure Policy - Require NSGs on all subnets except GatewaySubnet

azurepolicy-nsgOnSubnets.json

{
    "type": "Microsoft.Authorization/policyDefinitions",
    "name": "enforce-nsg-on-subnet-except-gatewaysubnet",
    "properties": {
        "displayName": "NSG X on every subnet except GatewaySubnet",
        "description": "This policy enforces a specific NSG on every subnet except the GatewaySubnet",
        "mode": "All",
        "parameters": {},
        "policyRule": {
            "if": {
                "allOf": [
                    {
                        "field": "type",
                        "equals": "Microsoft.Network/virtualNetworks/subnets"
                    },
                    {
                        "anyOf": [
                            {
                                "field": "name",
                                "notequals": "GatewaySubnet"
                            }
                        ]
                    },
                    {
                        "field": "Microsoft.Network/virtualNetworks/subnets/networkSecurityGroup.id",
                        "exists": "false"
                    }
                ]
            },
            "then": {
                "effect": "audit"
            }
        }
    }
}