Explanation of the error:
You may see this Salesforce validation error when you try to deploy a Workflow, where one org is Enterprise and the other is Professional.
Workflows are only available in Enterprise level orgs and upwards unless purchased from Salesforce as an add-on to a Professional org.
Resolution:
This error occurs because we are trying to query Salesforce for a type of metadata that is not supported.
You can get past this by removing the Workflow metadata type from your metadata filter, and then re-running the comparison.
Disclaimer: This error is returned by Salesforce directly, rather than Gearset. Even so, we offer guidance based on our combined experience with the Metadata API. Where possible, we try to help guide you to fix or avoid this error. In the case that this isn't possible, we may need to direct you to Salesforce support for further clarification.