Update gateway-migration.md

Author: MekaylaMoore

articles/expressroute/gateway-migration.md

@@ -64,6 +64,7 @@ The guided gateway migration experience has the following limitations:
 - **Private Endpoint Connectivity**: Private endpoints (PEs) connected via ExpressRoute private peering may experience **connectivity issues** during migration. Refer to guidance on mitigating these issues in the Private endpoint connectivity documentation. [Private endpoint connectivity](expressroute-about-virtual-network-gateways.md#private-endpoint-connectivity-and-planned-maintenance-events).
 - **Legacy Gateways**: ExpressRoute gateways created or connected to circuits in **2017 or earlier** aren't supported.
 - **Unsupported SKUs**: Gateways using the **"default" SKU** aren't eligible for migration. To check the migration eligibility of your Gateway, there should be an Advisor notification.
+- **Incompatible dedicated circuit**: Gateway migration can't proceed with a dedicated Hardware Security Module (HSM) connected to the virtual network. To proceed with the migration, deallocate the dedicated Hardware Security Module (HSM). For detailed troubleshooting steps, see [Troubleshoot Dedicated HSM](/azure/dedicated-hsm/troubleshoot).
 
 For detailed troubleshooting errors and best practices, see [Troubleshooting Gateway Migration](gateway-migration-error-messaging.md).