This Salesforce validation error that Gearset is surfacing is likely to occur on the DuplicateRule metadata type during an org to org, or Git (branch) to org deployment.
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On below example we're seeing the error:
​<xxx> is not a valid condition field for : <yyy> - where <xxx> refers to a specific condition field, while <yyy> refers to a Custom object component that failed the validation (in this specific example it is "Sub Account Form").
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Based on the information provided by Salesforce, this error can happen when you deploy changes to metadata that contains a DuplicateRule "with a condition that filters on a field that has a duplicate API name of a field on another object."

Remove the condition from the Duplicate Rule (in your source org), and then try to validate and deploy in our app.
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After a successful deployment, you can manually add the condition to the Duplicate Rule in the target org.

If you need more information on this error, such as steps to reproduce it, please check:
​Salesforce Known Issues