Ensure both GatewayIP and NoGateway cannot be set in subnets
diff --git a/openstack/networking/v2/subnets/errors.go b/openstack/networking/v2/subnets/errors.go
index 0db0a6e..d2f7b46 100644
--- a/openstack/networking/v2/subnets/errors.go
+++ b/openstack/networking/v2/subnets/errors.go
@@ -7,7 +7,8 @@
 }
 
 var (
-	errNetworkIDRequired = err("A network ID is required")
-	errCIDRRequired      = err("A valid CIDR is required")
-	errInvalidIPType     = err("An IP type must either be 4 or 6")
+	errNetworkIDRequired    = err("A network ID is required")
+	errCIDRRequired         = err("A valid CIDR is required")
+	errInvalidIPType        = err("An IP type must either be 4 or 6")
+	errInvalidGatewayConfig = err("Both disabling the gateway and specifying a gateway is not allowed")
 )