There are a few common causes for this error, please use the one which best applies for you below.
Explanation of the error:
This Salesforce error that Gearset is surfacing is usually encountered when trying to update the externalSharingModel on two custom objects at the same time. Salesforce can only run one sharing calculation at a time, resulting in this error.
Resolution:
To work around this issue try deploying the objects separately.
Explanation of the error:
Simultaneously updating the sharingModel field for an object whilst adding a new owner-based sharing rule as this isn’t supported in Metadata API.
Resolution:
Add an owner-based sharing rule when the org-wide default is public, and then update this on your sharingModel; this results in a single sharing recalculation rather than multiple. Then you can deploy a criteria-based or guest user sharing rule together with your changes to the sharingModel field using the Metadata API.
Further examples of this Salesforce validation error can be found here:
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.