On Demand Migration (ODM) does not migrate Gateways, these are only mapped or matched between the source and target tenants.
For the mappings to be completed any source tenant Gateways must be created in the target tenant and configured to access the same source systems prior to completing the matching or mapping process. This is completed using the
Microsoft Power BI Data Gateway installation process and any issue should be verified by Microsoft support.
- Any Gateways that are used by Connections that would be included in Workspace migrations do require the Target Gateway to be listed in ODM.
- If the Target Gateway is not listed there will be errors and warnings in the migration task
- Gateways in the target must be in an Online state for Connections migrations.
Gateway not mapped:
Gateway mapped: 
The Gateway tasks will fail for the following conditions:
- Gateways have not been created in the target tenant
- Names used for the Gateways are incorrect
- ODM Power BI access group has not been added to a target Gateway
In the event of errors and the above has been verified, review the Events for the Mapping/Matching task and search the ODM public
knowledgebase