code: def add_mobile_gateway(self, gateway):
          
          try:
              gateway = gateway.vpn.internal_gateway.href # Engine
          except AttributeError:
              gateway = element_resolver(gateway) # External Gateway
          
          self.make_request(
              PolicyCommandFailed,
              method='create',
              resource='mobile_gateway_node',
              json={'gateway': gateway,
                    'node_usage': 'mobile'})
docs: Add a mobile VPN gateway to this policy VPN. 
          Example of adding or removing a mobile VPN gateway::
          
              policy_vpn = PolicyVPN('myvpn')
              policy_vpn.open()
              policy_vpn.add_mobile_vpn_gateway(ExternalGateway('extgw3'))
              
              for mobile_gateway in policy_vpn.mobile_gateway_node:
                  if mobile_gateway.gateway == ExternalGateway('extgw3'):
                      mobile_gateway.delete()
              policy_vpn.save()
              policy_vpn.close()
          
          :param Engine,ExternalGateway gateway: An external gateway, engine or
              href for the mobile gateway
          :raises PolicyCommandFailed: could not add gateway