The FIELD_CUSTOM_VALIDATION_EXCEPTION error can occur during a data deployment if you have added a new custom validation rule or an Apex trigger in your target org. 

Gearset’s data loader will throw this error when you try to deploy records in the source org that fail a custom validation rule or trigger an error by calling the addError() method in your target org. When you add a new validation rule or a trigger to your orgs, Salesforce doesn’t check that all of your existing data complies with the new rule. For instance, you might have added a custom validation rule to both your source and target orgs before attempting to deploy records that fail the new rule. The error then occurs because Gearset cannot insert these invalid records into the target.

The straightforward way of avoiding this error is to disable the custom validation rule or Apex trigger in your target org before running the deployment. Gearset can help with that. A more general piece of advice is to check that a new validation rule or trigger is valid for your existing data before adding it to your org. Alternatively, for the purpose of the deployment, you may choose to filter and exclude the record(s) that give rise to the error.