Explanation of the error
This Salesforce error that Gearset is surfacing is usually encountered when deploying a picklist or a new value for a picklist.
This is because the metadata that Salesforce uses doesn’t quite line up to what the Salesforce UI would lead you to believe. An example of this is picklist values on standard fields; for instance, the values of the Account Source field on the Account standard object.
The Salesforce UI makes it look as if the field actually stores the value, but the reality is that the values are stored in another metadata type called a Standard Value Set and the actual field definition for Account Source is only a reference to the Standard Value Set.
Resolution
To resolve this issue try running a comparison that includes the Standard Value Set, Global Value Set and Custom object metadata types in a custom filter. You can then filter the Gearset result and find the changes you're interested in.
This validation error is also linked to Picklist value: <xxx> in picklist: <yyy> not found.
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.